c++/modules: use optimized crc32 from zlib

c++/modules: use optimized crc32 from zlib

The current implementation of bytes::calc_crc computes the checksum one
byte at a time which turns out to be quite slow, accounting for 15% of
streaming in time for a modular Hello World.  We have a crc32_unsigned
version that processes 4 bytes at a time which we could use here, but
since we bundle zlib we might as well use its highly optimized crc
routines that can process up to 32 bytes at a time.

So this patch makes us use zlib's crc32 in this hot code path.  This
reduces stream in time for a modular Hello World by around 15% for me
with a release compiler.

gcc/cp/ChangeLog:

	* Make-lang.in (CFLAGS-cp/module.o): Add $(ZLIBINC).
	* module.cc: Include <zlib.h>.
	(bytes::calc_crc): Use crc32 from zlib.
	(bytes_out::set_crc): Use crc32_combine from zlib.

Reviewed-by: Jason Merill <jason@redhat.com>