Allow `ValueSet`s to be constructed with slices in addition to arrays

[maddiemort]

Motivation

I'm working on an implementation of Subscriber that needs to be able to store (in a form as close as possible to the original) the inputs to its methods, and reconstruct them later to be passed to another Subscriber. For the most part, this works really well, but the biggest sticking point by far came from trying to reconstruct a ValueSet from a stored list of (Field, Value) pairs.

The core of the problem is the ValidLen trait and its usage by FieldSet::value_set(). In order to call that function, you have to be able to provide a statically-sized array, because that's the only type that implements ValidLen.

While, in theory, constructing an array for my use case should be just about possible (although the tricks necessary to do so likely involve a macro and the reimposition of an arbitrary length limit), lifetime issues (that appear to result from a call to <[_; N]>::try_from()) make it impossible. I'm going to construct a minimal example of this and investigate further at some point, but in the meantime:

Solution

Since #2508 removed the limitation on array length here, there doesn't seem to be a drawback to simply adding an implementation of ValidLen for slices. They already meet the Borrow supertrait requirement, so no additional machinery is needed.

This makes this API trivially easy to call for my use case, and I can't think of any associated drawbacks.

[maddiemort]

I'm not entirely sure why CI is failing, but I don't think it's related to my change.

[hawkw]

This change makes sense to me. The CI failure is due to a yanked dependency breaking our build with -Z minimal-versions, that's not related to this change.