fixed compile error UNITY_PRINT_EXEC_TIME(). Thanks @meslem12 !

Issue #317 - unity.h should include <stddef.h>

Added include of 'stddef.h' to 'unity_internals.h' if 'UNITY_EXCLUDE_STDDEF_H' is not defined. This adds compiler independent support for the 'NULL' macro.

Print execution time for each test when verbose mode enabled (Thanks @elliot-gawthrop, @shreyasbharath, and everyone who participated in this one!)

Fixed some grammar errors on docs (Thanks @Deryew !)

Fixed grammar errors and some sentences to make it easier to understand

Fix LESS_OR_EQUAL_MESSAGE asserts for HEX32/HEX64 (#337)

Added notes on _MESSAGE assertions in UnityAssertionsReference.md

Macros TEST_ASSERT_LESS_OR_EQUAL_HEX32_MESSAGE() and TEST_ASSERT_LESS_OR_EQUAL_HEX64_MESSAGE() need to be mapped to UNITY_TEST_ASSERT_SMALLER_OR_EQUAL_HEXnn() instead of UNITY_TEST_ASSERT_SMALLER_THAN_HEXnn()

Added notes, why _MESSAGE assertions do not support printf style formatting and how users can work around this limitation (see #331)

Flush unity output before a longjmp (Thanks! this is a good change)

Flush the unity stdout buffer before calling TEST_ABORT().
This is because if TEST_PROTECT() has not previously been called this will cause a segmentation fault and the stdout buffer will fail to print

Although the segmentation fault will still occur, the error that caused it will at least be displayed

Add support for :mock_suffix

Adds support for :mock_suffix when generating mock setup and teardown
functions. Also documents both prefix and suffix in the helper script
guide.

Re: Fix custom UNITY_OUTPUT_FLUSH and add tests #287

Bugfixes and refactoring in parse.rb (thanks @farrrb and @jeremyhannon )

Fixed unity fixture output and added methods for each of the different outputs. Added documentation.
Fixed some whitespaces.
Refactored class_name to class_name_idx (expressiveness).
Refactored the xml output to methods (extensibility).

- Added more expressiveness to the code
- Fixed some of the rubocop hints

Refactored the os specific settings, it is now possible to convert both styles on every system (and even mixed)

- Fixed stdout output if fixture is active
- Refactored the state manipulation of @test_suite and moved it completely into test_suite_verify()

 void flushSpy(int c) {}
to:
 void flushSpy(void) {}

Some minor changes for parse_output.rb (Thanks @farrrb  and @jeremyhannon !)

MISRA 19.10: surround each macro param with parens.

MISRA 2004 rule 19.10: inside macros, surround each parameter usage with parentheses.