Issue Description:

Slice does not implement withContiguous[Mutable]StorageIfAvailable, which means it falls to the default implementation which, unconditionally nil s out, even when Base does provide a custom implementation.

This is most noticeable with Unsafe[Mutable]BufferPointer:

var a = UnsafeMutableBufferPointer<Int>.allocate(capacity: 4)
a[1...].withContiguousStorageIfAvailable { print($0) } // does not print

Providing a good implementation on Slice is somewhat tricky because we are lacking constraints on the index type of Base, but it would be nice to improve this.