Extract simple integer math into its own module

.gitignore

Makefile.am

src/common/util.c

src/common/util.h

src/include.am

src/lib/container/.may_include

src/lib/container/bloomfilt.c

src/lib/crypt_ops/.may_include

src/lib/crypt_ops/crypto_pwbox.c

src/lib/intmath/.may_include

src/lib/intmath/addsub.c

src/lib/intmath/addsub.h

src/lib/intmath/bits.c

src/lib/intmath/bits.h

src/lib/intmath/cmp.h

src/lib/intmath/include.am

src/lib/intmath/muldiv.c

src/lib/intmath/muldiv.h

src/rust/build.rs