Minor, simplify method
Minor, fix/suppress warnings in codegen
Delete Enum.name()/ordinal() JVM intrinsics

Bytecode is the same if they're generated as is from descriptors

+ Some minor code style improvements
Fix KotlinBuiltIns.isUnit

It didn't check type's nullability
Fix codegen for closures returning 'Unit?'

Only do a checkcast when we need to coerce Object to Unit: the code was
hopefully type-checked so that it'll only be necessary when the value is either
Unit.VALUE or null
Rename KotlinBuiltIns.isAny -> isAnyOrNullableAny

Reduce confusion, prevent bugs
Delete unused code
Utilize existing mapType methods

Fix warnings related to nullable CallableDescriptor.getReturnType()