Deprecate `__int128`, `__uint128`, `__int128_t`, `__uint128_t`

[tgross35]

These types are only available on aarch64, but similar to int8_t, int16_t, etc. they don't have much purpose. We should deprecate them and remove in 1.0, replacing the single in-crate use with i128.

[sanstzu]

Hi @tgross35, I would like to attempt to do this issue.

[tgross35]

Assigned you, thanks! It should be pretty easy.