Simplify type mapping logic in CodegenBinding

- inline asmType to calling getAsmType, which does something more
- refactor getJvmInternalName to use getAsmType as well
- simplify getAsmType and fix a probable bug in mapping singletons nested in
  enums (which wasn't reproduced, though a test is added)
- delete unnnecessary ASM_TYPE recording for enum entries in
  CodegenAnnotatingVisitor