Add unchecked_slice

[tcbrindle]

As the name suggests, this is just like slice<T> except that it does no bounds checking -- so most of its operations cause UB if their preconditions are not met. Its iterators are just raw pointers. There is no unchecked_slice::at(), because it's not clear what semantics it should have. Like slice, unchecked_slice is not publicly copyable or movable.

With this in place, we can give slice<T> a public member named unchecked of type unchecked_slice<T>, and implement the (bounds checking) slice methods on top of that. Array pointers (tcb::pointer<T[]>) still deref to slice<T>& or slice<T> const& as before.

The idea is to make bounds checked operations the default, but allow users to explicitly opt in to using unchecked operations when necessary, in a way that is clear and obvious in source code. For example:

auto& slice = *ptr;

int i = slice[10]; // bounds checked
int j = slice.unchecked[10]; // explicitly not bounds checked

my_algorithm(slice.begin(), slice.end()); // bounds checked iters
my_algorithm(slice.unchecked.begin(), slice.unchecked.end()); // explicitly unchecked iters

my_range_algorithm(slice); // uses checked iters
my_range_algorithm(slice.unchecked); // uses unchecked iters