tuple.
This closes #649

Squashed commit of the following:

commit 96f415bc1744533e96a74bcc85016ff5df65d06e
Author: Karthikeyan Jambu Rajaraman <karthi.jrk@gmail.com>
Date:   Thu Apr 21 15:08:49 2016 -0700

    Don't call prepare for execution if no generator is registered

commit 5318e9214a3bbc237c0d79017cdef02538c4e711
Author: Karthikeyan Jambu Rajaraman <karthi.jrk@gmail.com>
Date:   Thu Apr 21 11:35:22 2016 -0700

    Adding if codegen protection in starting active manager and while
    deleting manager

commit 12b0045d51a5b056b32a6cc9ef370e6d9af315fd
Author: Karthikeyan Jambu Rajaraman <karthi.jrk@gmail.com>
Date:   Fri Apr 15 14:33:55 2016 -0700

    Updating Readme for Codegen GPDB Function

commit d17119c5d8ce9a706b3321d5e7ad41f419fddbdc
Author: Karthikeyan Jambu Rajaraman <karthi.jrk@gmail.com>
Date:   Wed Apr 13 13:25:14 2016 -0700

    Add documentation for how to codegen in README.md

commit d711be4bb044592e7f97bf31ceb5f32337ad6d03
Author: Karthikeyan Jambu Rajaraman <karthi.jrk@gmail.com>
Date:   Wed Apr 13 11:19:03 2016 -0700

    Fixing doxygen namespace addtogroup

commit e34131bcc9bb3d36c5e8995e1a0efe11fbd55417
Author: Karthikeyan Jambu Rajaraman <karthi.jrk@gmail.com>
Date:   Wed Apr 13 11:17:32 2016 -0700

    Changing CodeGen to Codegen

commit bedfcca80b5f1ff796783ca7c2035d920ae86c4e
Author: Karthikeyan Jambu Rajaraman <karthi.jrk@gmail.com>
Date:   Wed Apr 13 10:51:39 2016 -0700

    Fix cpplint error

commit 32f89feedba655e1242529c78a54faf01ddf32bf
Author: Karthikeyan Jambu Rajaraman <karthi.jrk@gmail.com>
Date:   Wed Apr 13 10:48:49 2016 -0700

    Change regular version of generated to regular version of target in comment

commit d1da2824c08e34eb7139b08cd9d2aff19bf008e7
Author: Karthikeyan Jambu Rajaraman <karthi.jrk@gmail.com>
Date:   Wed Apr 13 10:42:09 2016 -0700

    Made GetFunctionPointer and CreateFunction to take typedef of function type as only public api

commit 9f48f77bd9fc50c7d29a4e8f7700452ef88dc79f
Author: Karthikeyan Jambu Rajaraman <karthi.jrk@gmail.com>
Date:   Tue Apr 12 17:18:24 2016 -0700

    Rename doCodegeneration to GenerateCodeInternal

commit 9884d07f4c6ab08d9ccca8d07b80974081cda1fa
Author: Karthikeyan Jambu Rajaraman <karthi.jrk@gmail.com>
Date:   Tue Apr 12 17:15:55 2016 -0700

    fix cpplint error

commit c21b3bee3e289675fc13017688269b40e26efa11
Author: Karthikeyan Jambu Rajaraman <karthi.jrk@gmail.com>
Date:   Tue Apr 12 17:08:16 2016 -0700

    Update comments

commit f016bb582af65e84def1b55917017008e292d45b
Author: Karthikeyan Jambu Rajaraman <karthi.jrk@gmail.com>
Date:   Tue Apr 12 17:01:19 2016 -0700

    Adding assert in static setToRegular function

commit 93a791a4ec415313933dd5b7c1ae17dbdd6193a6
Author: Karthikeyan Jambu Rajaraman <karthi.jrk@gmail.com>
Date:   Tue Apr 12 16:49:15 2016 -0700

    Removing decltype in Slotdeformtype codegen since we know the functype

commit ed95193d0991cb24fd68f6d9a473de630f297b0c
Author: Karthikeyan Jambu Rajaraman <karthi.jrk@gmail.com>
Date:   Tue Apr 12 16:45:49 2016 -0700

    Fixed compiler / unit test failure
    Changed InitCodeGen to InitCodegen

commit 3c95c9efdd4336e100a86748b0e87d79f07bcc11
Author: Karthikeyan Jambu Rajaraman <karthi.jrk@gmail.com>
Date:   Tue Apr 12 16:28:24 2016 -0700

    Changing CodeGen back to Codegen while cherry-picking

commit 4e24ef22bcf54edc00c2a0d1f9f9703364cdf5ba
Author: Karthikeyan Jambu Rajaraman <krajaraman@pivotal.io>
Date:   Tue Apr 12 11:08:42 2016 -0700

    Changing auto to actual type

commit 1d92fc190dda3ec866edc83d56d9eca9fe90a63a
Author: Karthikeyan Jambu Rajaraman <krajaraman@pivotal.io>
Date:   Tue Apr 12 10:44:49 2016 -0700

    fix indent in codegen_utils

commit ea6fa787a2332f94b8d20d24825ba1f0a240d02e
Author: Karthikeyan Jambu Rajaraman <krajaraman@pivotal.io>
Date:   Mon Apr 11 17:25:53 2016 -0700

    Adding codegen guc to wrapper function so related function not executed when codegen guc is disabled

commit f1cd7d80d8d81f3565f3ab12cb94a3c4cc58c44a
Author: Karthikeyan Jambu Rajaraman <krajaraman@pivotal.io>
Date:   Mon Apr 11 17:13:24 2016 -0700

    Changing bool to unsigned int in codegen_wrapper.h

commit 7ef462540a312d077b332c732c125c55dedb6b14
Author: Karthikeyan Jambu Rajaraman <krajaraman@pivotal.io>
Date:   Mon Apr 11 16:59:08 2016 -0700

    Fix based on lint complain

commit ddc55cec0d579a03cb9e2303efa6d74f5183979f
Author: Karthikeyan Jambu Rajaraman <krajaraman@pivotal.io>
Date:   Mon Apr 11 16:19:48 2016 -0700

    Removing commented code from unittest

commit 28b8886ed2bfdc854aa38f871a41536c1e6cef29
Author: Karthikeyan Jambu Rajaraman <krajaraman@pivotal.io>
Date:   Mon Apr 11 16:17:45 2016 -0700

    Chaging auto in for to actual type

commit ad8d78e07e9b789f4d8ae3c6ccc3cea1a05089c0
Author: Karthikeyan Jambu Rajaraman <krajaraman@pivotal.io>
Date:   Mon Apr 11 16:13:21 2016 -0700

    * Handle return value of SetToGenerated from PrepareForExecution
    * Change size_t to unsigned integer

commit d46ee8be414b3ac2a9ea7ef899f29cef3df145c7
Author: Karthikeyan Jambu Rajaraman <krajaraman@pivotal.io>
Date:   Mon Apr 11 15:30:40 2016 -0700

    Introducing template function to do enroll for any type of generator

commit 83428605f906e28d0fc994dcc311e0cc3e6d9e12
Author: Karthikeyan Jambu Rajaraman <krajaraman@pivotal.io>
Date:   Mon Apr 11 14:41:11 2016 -0700

    Changing manager to take module name as argument

commit b88aa40f89860e7c15503365ea304d2392f71cda
Author: Karthikeyan Jambu Rajaraman <krajaraman@pivotal.io>
Date:   Mon Apr 11 14:27:31 2016 -0700

    adding more comments to SlotDeformTupleCodeGenInfo and unique counter

commit df1b704548fcbc1b43524064eb2f024995eea865
Author: Karthikeyan Jambu Rajaraman <krajaraman@pivotal.io>
Date:   Mon Apr 11 14:08:34 2016 -0700

    Change test target to have ext .t and adding gitignore for that extension

commit 9ee3521ca687c4ea7f13d15893f47146ecdb5277
Author: Karthikeyan Jambu Rajaraman <krajaraman@pivotal.io>
Date:   Mon Apr 11 13:59:57 2016 -0700

    Add target_include_directories for test library header file

commit effb7dc165b4c7094731dc39e0872741b28b2e6a
Author: Karthikeyan Jambu Rajaraman <krajaraman@pivotal.io>
Date:   Mon Apr 11 13:36:50 2016 -0700

    Move all possible ifdef from gpdb to codegen_wrapper based on pull request comments

commit 989b96a91c1e843577b5f24623d1e4a6b810394c
Author: Karthikeyan Jambu Rajaraman <krajaraman@pivotal.io>
Date:   Fri Apr 8 10:43:17 2016 -0700

    fixing comments for GetOrigFuncName in codegen_interface

commit 665de065c34ea13abcec8c5f3a3336558dae501f
Author: Karthikeyan Jambu Rajaraman <krajaraman@pivotal.io>
Date:   Fri Apr 8 10:27:31 2016 -0700

    Disabling codegen guc by default

commit ec34513e86dea95bd36ee528724273c1c19e7062
Author: Karthikeyan Jambu Rajaraman <krajaraman@pivotal.io>
Date:   Thu Apr 7 16:28:43 2016 -0700

    Fixed comments space/tab so it aligned in different editor

commit 35a60fe2fd053a5babd1820302afad95c5c2635f
Author: Karthikeyan Jambu Rajaraman <krajaraman@pivotal.io>
Date:   Thu Apr 7 16:12:08 2016 -0700

    Fixing coding style based on cpplint

commit 9477dcf6c0a7ffcca77b9f2c62c7340d13f474aa
Author: Karthikeyan Jambu Rajaraman <krajaraman@pivotal.io>
Date:   Thu Apr 7 13:37:01 2016 -0700

    Adding USE_CODEGEN ifdef

commit 142ffc1416341a3ac1d0fe37a83577168f2ec166
Author: Shreedhar Hardikar <shardikar@pivotal.io>
Date:   Tue Apr 5 22:21:19 2016 +0000

    Make codegen work with GPDB unit test framework

commit 9dca9507dd933a5517bee3ca34615c09c23c1728
Author: Shreedhar Hardikar <shardikar@pivotal.io>
Date:   Mon Apr 4 22:10:21 2016 +0000

    Add gpcodegen_mock.c

commit 41d515ff0342d3df3b0853ad91731f6d1d75843c
Author: Shreedhar Hardikar <shardikar@pivotal.io>
Date:   Mon Apr 4 21:46:34 2016 +0000

    Remove unneccessary header file includes in codegen

commit 14c5e14b9834197693b1ef56efa96386cfe049c5
Author: Foyzur Rahman <frahman@gmail.com>
Date:   Tue Apr 12 14:49:51 2016 -0700

    Integrate codegen framework and make simple external call to slot deform tuple
Signed-off-by: NKarthikeyan Jambu Rajaraman <karthi.jrk@gmail.com>
Signed-off-by: NFoyzur Rahman <frahman@gmail.com>