Add checking that a type only satisfies a constraint if it satisfies all of that constraint's equality and rewrite constraints.
Enforce the rule that `=` must be used in impls when specifying associated constant values rather than `==`.
Turn off the pre-#2173 single-step equality behavior. This is getting somewhat ahead of the approved design, but it's a one-line change to restore the old behavior.
Fix `CARBON_CHECK` to handle top-level `,`s in its argument, such as may happen in template argument lists, as this change introduces such a check.
Richard Smith
4b679510e7