internal refactoring: cleanup type relations

[lcnr]

we should try to clean up our type relations further:

• try to unify Equate and Sub to make them closer to nll::TypeRelating
• merge Lub and Glb into the same (maybe generic) type relation, prolly just with a field
• reuse more components, e.g. instantiating infer vars should be shared, not only generalize
• completely merge Equate/Sub and nll:TypeRelating

I intend to look into this myself but would also appreciate some help. This is a quite involved and sometimes subtle area however and I don't have the capacity for in-depth mentoring rn

[ShoyuVanilla]

try to unify Equate and Sub to make them closer to nll::TypeRelating

It seems to be done by #121462, I guess?

[ShoyuVanilla]

completely merge Equate/Sub and nll:TypeRelating

And nll::TypeRelating is moved into rustc_borrowck in #121321