* slice: fix memory semantics
* slice: prefer passing slice by value
This lets us do nice things like coercing Slice<T> to Slice<const T>, etc.
* slice: fix unit tests
* slice: fix copy/assignment, hash function
Don't attempt to define a copy constructor/assignment operator which would
convert a constant type to a mutable type.
Additionally, fix the hash function such that we're hashing U instead of const
U.
