`[T]::as_mut_array` and `hybrid_array::AsArrayMut::as_array_mut`

[tarcieri]

Rust 1.93 landed the following stable API: https://doc.rust-lang.org/stable/std/primitive.slice.html#method.as_mut_array

I noticed our AsArrayMut should probably be AsMutArray among other things. Maybe we could release a hybrid-array v0.5 prior to stabilization which cleans some things up. Too late!

I also thought it might be interesting to support an API like that: returning an optional array reference if the slice is sized appropriately. We also have a few other "slice helper" methods currently defined on Array (where they can be const fn) but perhaps we could use a SliceExt as well with extension methods for slices (so long as the names don't clash, which seems hard, maybe we could look at the *array* methods on slices and make parallel harray methods?) Edit: opened #168

[tarcieri]

#172 added SliceExt::as_(mut_)hybrid_array

[tarcieri]

#135 added AsArrayMut::as_array_mut (before this issue was opened). I think this can be closed.