merge generalizer impls

[lcnr]

we currently have 3 different implementations of "generalization":

https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/combine/struct.Generalizer.html

https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/nll_relate/struct.TypeGeneralizer.html

https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/combine/struct.ConstInferUnifier.html

These 3 should have the same behavior and should be merged.

cc #109813 where we have to do the same change in each of the generalizers.