Change interface for IAfterCompileContext and execute AfterCompile event

[davidfowl]

The after compile context should give access to the assembly and symbol (pdb/mdb) stream.

[AssemblyNeutral]
public interface IAfterCompileContext
{
    CSharpCompilation CSharpCompilation { get; set; }
    IList<Diagnostic> Diagnostics { get; }
    Stream AssemblyStream { get; set; }
    Stream SymbolStream { get; set; }
}

Linked to #39

[muratg]

@lodejard @davidfowl do you guys want this for RC1? do either of you have any cycles?

[davidfowl]

I did this already

[davidfowl]

4a7994d