go1.8 changed the way anonymous struct names are generated for DWARF.

* tests: update to cope with go1.7 SSA compiler

* de-vendored golang.org/x/debug/dwarf

We need our own tweaked version

* dwarf/debug/dwarf: always use the entry's name attribute

Using the name attribute leads to better type names as well as fixes
inconsistencies between 1.5, 1.6 and 1.7.

* proc: Updated loadInterface to work with go1.7

go1.7 changed the internal representation of types, removing the string
field from runtime._type.
Updated loadInterface to use the new str field.

If we are unable to correctly guess how a type is serialized in
debug_info let the user specify it directly.

Workaround for #455

* service/api: Removed unused fields of service/api.Function

* proc/eval: Set return variable name to input expression

* all: fine-grained control of loadValue for better variable printing

Makes proc.(*Variable).loadValue loading parameters configurable
through one extra argument of type LoadConfig.
This interface is also exposed through the API so clients can control
how much of a variable delve should read.

New API version with better backwards compatibility plus mechanism to
select the API version that a headless instance should use.

Adds service/test/cmd/typecheckrpc.go to type check the RPC interface.

proc.(*Process) methods are not thread safe, multiple clients
connecting simultaneously to a delve server (Issue #383) or a even
a single over-eager client (Issue #408) can easily crash it.
Additionally (Issue #419) calls to client.(*RPCClient).Halt can
crash the server because they can result in calling the function
debug/dwarf.(*Data).Type simultaneously in multiple threads which
will cause it to return incompletely parsed dwarf.Type values.

Fixes #408, #419 (partial)

Moved the code in testvariables2 and testvariables4 into
testvariables3, renamed testvariables3 into testvariables2

Past the maximum recursion depth maps shouldn't be loaded at all,
adding map children and not loading them breaks assumptions in
the prettyprinter.

Fixes #406

Typedefs that resolve to slices are not recorded in DWARF as typedefs
but instead as structs in a way that there is no way to know they
are really slices using debug/dwarf.
Using golang.org/x/debug/dwarf instead this problem is solved and
as a bonus some types are printed with a nicer names: (struct string
→ string, struct []int → []int, etc)

 Fixes #356 and #293

Fixes #320

Go1.6 changed the behaviour of go/constants.

The concrete type of an interface only contains the abbreviated
package name, we must construct a map from package names to package
paths to be able to resolve the concrete type of an interface.

It's was implemented unintentionally and the unintentional
implementation doesn't work and causes a crash.

supported: len, cap, imag, real, complex

Fixes #288

Use m[n:] to skip the first n keys of a map
Map indexing is implemented with a linear scan

Implements #61, #122

Pretty print will print them with the format:
chan <element type> <queued elements>/<queue size>

Supported operators:

- All (binary and unary) operators between basic types except <-,
++ and -- (includes & to take the address of an expression)
- Comparison operators between supported compound types
- Typecast of integer constants into pointer types
- struct members
- indexing of arrays, slices and strings
- slicing of arrays, slices and strings
- pointer dereferencing
- true, false and nil constants

Implements #116, #117 and #251

The new contents of api.Variable are documented in
proc/variables.go.

Implements #243