Remove dependency on GenASM for structure builder.

We relied on genJVM, and later on genASM, to retrieve method descriptors for 
method parameter types in the structure builder. That part of the code proved
to be a bit unstable, and there is no reason why we should continue to rely on
it.

This commit implements the same functionality on our side.

* Sorry for the whitespace changes in `ScalaJavaMapper`. The only meaningful
changes are in `mapParamTypeSignature`.
* the current code still relies on scalac backend, just one step earlier
(icode).