PyUnicode_AsUnicode function signature mismatch on UCS2/UCS4 #408

Signatures of the functions are different.

UCS2:
internal unsafe static extern char* PyUnicode_AsUnicode(IntPtr ob);

UCS4:
internal unsafe static extern IntPtr PyUnicode_AsUnicode(IntPtr ob);

Leading to weird hacks in the code such as 1 and 2

Consolidate signature to IntPtr and remove PyUnicode_AS_UNICODE which does the same thing.