Obj-C built-ins prototype

Add default import path for built-in classes, resolve these classes via
JavaDescriptorResolver. This is a temporary hack, built-ins probably need to be
implemented in Kotlin later
Support Pointer<T> in ObjCTypeResolver

"void *" maps to Pointer<*>, and any other pointer maps to the Pointer of the
corresponding mapped pointee type
Drop indexing Obj-C ivars

They don't seem to be needed at all
Support Pointer<Char> in Obj-C+Java
Support string arguments in Obj-C methods

Pointer.pointerToString allocates a new native char array and fills it with the
chars of the given String
Support pointer as Obj-C method return type

ObjC class is moved closer to Pointer to invoke its package constructor, which
shouldn't be visible outside; and renamed, because it's a bunch of helpers to
Native.

Add tests for returning strings and NSStrings
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