Update to use the latest Roslyn version and add support for C# 7

[danroth27]

[Eilon]

Two main things to do:

1. Reference the newer version of Roslyn
2. Evaluate all new language features to see what works and what doesn't.
   • Even for things that work, need to add tests for new syntax features to make sure it never breaks.

[Eilon]

Related issue w/ tuple syntax in generics: #1070

[NTaylorMullen]

Evaluate all new language features to see what works and what doesn't.

Went through the features and they all parse correctly (yay!).

The work for this issue is then:

1. Reference the newer version of Roslyn
2. Add tests for C# 7 features

[rynowak]

FYI checking this in is blocked right now on us having the right versions for our tooling to consume.