fix: long != intnat
Merge pull request #177 from fdopen/master

fix: long != intnat
Move the cptr type from Static to Ctypes_raw.
Rename Ctypes_raw to Ctypes_ptr.
Turn Ctypes_ptr.cptr into an abstract type.
Rename Ctypes_ptr.PtrType to Ctypes_ptr.Raw.
Ctypes_bigarray.address ~> unsafe_address, and remove an unused parameter.
Eliminate (most) calls to unsafe_raw_addr.
Implement to_voidp and from_voidp in terms of castp; remove an unused cast.
Some comments on the inherent lack of safety of raw_address_of_ptr.