In tstate::auxiliary::constraint, find a way to "freeze" the descs field

[catamorphism]

This is a supreme yak-shave, but the type tstate::auxiliary::constraint has a mutable vector (dvec) field called descs, but it only needs to be mutable during the collect_locals part of typestate. Ideally collect_locals would use this type internally, then return a different type that has an immutable vector field. But I'm not sure if there's a good way to turn a dvec into an immutable vector.

Contingent on typestate not getting removed.

[nikomatsakis]

Not relevant as of commit 41a21f0, which removed current incarnation of typestate.