Rollup merge of #71973 - lcnr:lazy-norm, r=nikomatsakis

Lazy normalization of constants (Reprise)

Continuation of #67890 by @skinny121.

Initial implementation of #60471 for constants.

Perform normalization/evaluation of constants lazily, which is known as lazy normalization. Lazy normalization is only enabled when using `#![feature(lazy_normalization_consts)]`, by default constants are still evaluated eagerly as there are currently.

Lazy normalization of constants is achieved with a new ConstEquate predicate which type inferences uses to delay checking whether constants are equal to each other until later, avoiding cycle errors.

Note this doesn't allow the use of generics within repeat count expressions as that is still evaluated during conversion to mir. There are also quite a few other known problems with lazy normalization which will be fixed in future PRs.

r? @nikomatsakis

fixes #71922, fixes #71986

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