If a type is mapped to Pointer, consider it a pointer

Otherwise, Pointers end up being passed to the methods declared
native, which won't work.
Treat <bitfield>s the same as <enumeration>s

They look almost exactly the same, the only difference seems to be
that while <enumeration>s allow for arbitrary integers as values
for their members, <bitfield>s only list single-bit flags. In BridJ,
enumeration members can be ORed together like flags, so the way we
map <enumerations> should work for <bitfield>s as well.
Update comment about parsing <alias>es
Fix generated methods returning Pointer<Pointer<something>>

Pointer.class was previously generated for these as the second
argument of Pointer.pointerTo() in pretty wrappers, which yields
a return type of Pointer<Pointer>, which is incompatible with
Pointer<Pointer<something>>. JNAerator does the same thing with
these return types.