is_sorted and is_sorted_until algorithms

You're confusing two things:

1. If the algorithm mutates the sequence, it doesn't make sense to accept `initializer_list`s, and
2. If the algorithm returns an iterator, it doesn't make sense to accept rvalue ranges.

`is_sorted_until` doesn't mutate the sequence, so it should accept `initializer_list`s. It returns an iterator, so it shouldn't accept rvalue ranges (that is, the signature is wrong).