take_while should not be random-access

[tcbrindle]

This was one of the very first adaptors and I just adapted my ranges implementation without thinking about it too much. But how can we bounds check if we don't know where the end is?

[tcbrindle]

(An "after pred() has returned false" read isn't going to be UB as the access to the underlying sequence is still going to be bounds checked, but we should still catch this.)