Enable geom for i128, which is always available.

[michaelkirk]

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• [x] I added an entry to CHANGES.md if knowledge of this change could be valuable to users.

I'm not sure why we had a check before - i128 is defined on all platforms as part of core.

https://doc.rust-lang.org/rust-by-example/primitives.html https://doc.rust-lang.org/core/primitive.i128.html