Add ast compiler to fsc.fs

I have added a `compileOfAst` method to `fsc.fs`. This is essentially `mainCompile` minus the parsing stage, which can be used for typechecking and compiling user-transformed untyped ASTs.
@dsyme I dont think the dynamic assembly option works, I get an error about resources not being present.  Maybe I used the wrong options.  A straight dynamic assembly works ok, but not with the parsetree passed in.