Use native UInt128 in Dragonbox

[HertzDevil]

This allows CACHE to be defined in a slice literal eventually.

[straight-shoota]

This seems to be breaking compatibility with Crystal <= 1.2
https://github.com/crystal-lang/crystal/actions/runs/18777089311/job/53592899456

The error message is not very telling: Error: while requiring "prelude". But 128-bit literals were only introduced in Crystal 1.3.
So we can't use them here while keeping forward compatibility down to 1.0 (related: #12937).