MGCA: merge associated_const_equality feature gate into MGCA #150617
Description
The "point" of MGCA is effectively to introduce support for associated constants in the type system. The current impl of associated_const_equality allows for a bunch of things into the type system that we don't really want.
A core part of "good support for associated constants in the type system" is supporting T: Trait<ASSOC = 1>. We should therefore merge associated_const_equality into the more general min_generic_const_args feature.
It's also worth noting that right now associated_const_equality doesn't really work well without min_generic_const_args enabled as we've started enforcing that associated constants in the type system meet the requirements that min_generic_const_args introduces. So in some sense you already need MGCA to use ACE.
cc @fmease the de-facto owner of associated_const_equality I think. We talked about this a while back on zulip.
This would involve replacing all features().associated_const_equality() with features().min_generic_const_args() in the compiler, as well as switching over all the feature gates in the test suite. As well as moving the feature to the removed list of features: https://rustc-dev-guide.rust-lang.org/feature-gates.html#removing-a-feature-gate