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.

setup.py

@@ -9,6 +9,11 @@ from setuptools import setup
 
 ROOT = Path(__file__).parent.resolve()
 
+if sys.maxsize <= 2**32:
+    raise RuntimeError(
+        "aeg does not support 32-bit platforms; build with a 64-bit Python"
+    )
+
 libaegis_static = ROOT / "libaegis/zig-out/lib" / (
     "aegis.lib" if sys.platform == "win32" else "libaegis.a"
 )