Migrate public types into monodoc.dll

[joelmartinez]

After we publish a monodoc nuget (#228 / #246), we should move some types that are in the mdoc.exe project into monodoc.

This should include type forwarders ... though I highly doubt anyone has taken a dependency against the .exe.

Good candidates for the move:

• the formatter suite ... this includes signatures, slashdoc, filename, etc.