Fix cdef size_t size on Win64 (LLP64), require 64-bit builds

The generated cdef typedef'd size_t as unsigned long: 8 bytes on LP64
but only 4 on 64-bit Windows, where C's size_t is 8. cffi's import-time
struct layout verification then failed (aegis_raf_scratch.len: cdef
says 4, C compiler says 8). cffi pre-defines size_t/uint8_t with the
correct platform sizes, so don't typedef them.

Also hard-fail 32-bit build attempts in setup.py; 32-bit platforms are
no longer supported.

src/aeg/aegis_cdef.h

@@ -1,8 +1,5 @@
 /* This file is generated with tools/generate.py. Do not edit. */
 
-typedef unsigned char uint8_t;
-typedef unsigned long size_t;
-
 /* aegis.h */
 int aegis_init(void);
 int aegis_verify_16(const uint8_t *x, const uint8_t *y);