Vec const-ness

[tower120]

I don't know if this suggestion was raised before in this crate, but it looks like Vec::from_array can be implemented in const context.
Along with some other methods.

I think I can implement that, if you accept PR's.

[sgued]

Yes, PRs would be welcome for more constness ! Last I remember not many things could be made const due to compiler limitations, but I might be wrong and would love to be proven so by a PR :)

PRs introducing a breaking change would be delayed for a long time, so if you must make some breaking changes to make some functions const, please make that a separate PR from other non-breaking const improvements, that will speed-up merging the ones that can be released now.

[tower120]

I have managed to implement const from_array without structural changes.

But implementing const set_len() , capacity() requires their "specialized" Vec-specific versions.
This can be achieved either through adding separate set_len_const() capacity_const(), etc...
Either through making Vec a separate type with VecInner inside - this way we can forward most
methods to VecInner implementation, and then one-by-one move member functions const.

If we go with the second option - we could make macro that forward trait implementations to
VecInner - in case, if you have more types with fixed capacity that could go this way.
(I'm not familiar enough with heapless - maybe strings, or something like that - could be refactored this way later).

[tower120]

Just a follow up to a "second option". I see that you sometimes rely on coercion to a VecView. This can be replaced with trait IntoVecView. Then, in all places where you expect VecView arg, you work with impl IntoVecView - in analogy to IntoIterator. This way all functions will still work with both Vec and VecView.

Of course, these all are breaking changes...

UPDATE. On a second thought - we could just Deref to an inner VecView and redefine all const functions with the same names in Vec. This way self. will always lead to Vec implementations, and arg: &VecView - to VecView implementations. Don't know how "beautiful" it will look though...