Made most types internal
Added overloads and extension methods to get well-known symbols directly from the compilation instead of a semantic model
Refactored SymbolMap and added SymbolTransformation
Removed ToString overrides from metadata types that were only required for testing; tests now use Unquote instead of FSUnit and therefore don't require overloaded ToString() methods anymore; on the other hand, Unquote (or rather F#'s reflection library for discriminated unions which Unquote heavily uses) doesn't work with internal types, therefore all metamodel types have been made public again