Property tests for standard functions generates mostly big ints

[Acaccia]

In the property tests in property_tests.rs, the generated i128 for the property tests are mostly big integers, and don't really test that functions still works for smaller ints.

We should split the int128() into several prop_compose, to have more control on the size we want to get.

I would propose at least:

• small_uint128: i <= u64::MAX
• big_uint128: i > u64::MAX
• positive_int128
• negative_int128