move GeneratorAdapter.push() code directly into MethodWriter.visitLdcInsn()
It's fairly common that someone need to generate constants (e.g. Array
initializing etc...) But I don't like to package and use GeneratorAdapter only
for it's load method.
I think that the type check and constant optimisation (use ICONST_xxx, BIPUSH,
SIPUSH, LCONST etc...) is most useful in MethodWriter.visitLdcInsn().