Add support for 32 Bit platforms

[dbahrdt]

Currently we rely on sizeof(signed long) == sizeof(int64_t) in various places.
In order to support 32 Bit platforms we would need to change that.
So far I've found the following places:
Conversion:h: initalizers
ExtendedInt64z: mpz_fits_slong_p, mpz_get_si

possibly more.

[dbahrdt]

This is hopefully fixed in the ei64 traits branch. It at least compiles on Debian 32 Bit