Abstracts the int128 type and provides an native version, if available, or a implements it using a pair of int64_t's.
This is activated by setting the configuration flag --with-test-override-wide-multiply=int128_struct.
Tidy #includes of int128.h and int128_impl.h
After this commit, int128.h and int128_impl.h are included as follows:
- .c files which use int128 include int128_impl.h (after util.h)
- .h files which use int128 include int128.h (after util.h)
This list is exhaustive. util.h needs to included first because it sets
up necessary #defines.
Add int128_struct tests
This is a backport of secp256k1#1000