Error Index for E0207 needs to mention const, lifetime parameters

[Havvy]

As it is currently written, E0207 talks entirely about type parameters. Now that const parameters also need to constrain implementations (under the same rules described for types), they also need to be mentioned in the error explanation. Since other items don't require const parameters to be used at all, we might also want to specifically call out this difference here.

Furthermore, E0207 triggers if you use an unconstrained lifetime in an associated type, and this is completely unspecified by the error as well.

[ffmancera]

@rustbot claim