Minor refactoring of TImpl codegen

Extract some methods
Minor refactoring of TImpl codegen

Use early returns, rename some variables, etc
Minor, extract methods in TImpl codegen
Minor refactoring of TImpl codegen

jvmSignature.getAsmMethod() and functionOriginal are supposed to be the same
entity, so move jvmSignature out of the way to where it's needed
Fix mapGetterSignature/mapSetterSignature

In case of TRAIT_IMPL the incorrect this was written. Reuse the code from the
common mapSignature

 #KT-3413 Fixed
Rename some local variables in ImplBodyCodegen