Feature: Documentation Support, includes DotNet XML consumption

[TIHan]

Enable documentation for all top-level constructs using ///.

/// This is a method called `M`
M(x: int32): () =

/// A class named 'C'
class C

/// A property named 'X'
X: int32 get

Tooling should pick these up and show this information in the IDE.

We also need to support the consumption of DotNet XML documentation from the corresponding .xml files. This will require translation to OlyIL documentation constructs.