Feature Request: Allow viewing a `DenseSlotMap` as a slice of `T`s

[SludgePhD]

DenseSlotMap is already documented to store all values contiguously, so an AsRef<[T]> and AsMut<[T]> impl could be added to it to allow the user to actually access that contiguous storage. (an as_slice and as_mut_slice method that do the same thing would also be nice, to match what libstd containers do)

The motivation is simply that some algorithms may already be defined to operate on slices, and rewriting them to take an impl IntoIterator would add too much churn and/or no longer guarantee optimizations the slice version would receive (and having them take the DenseSlotMap or its iterator directly would cause needless API pollution with random types from dependencies). A good example is the salva library, which currently defines its own contiguous arena type.