use std::less instead of < for comparison

[Jarod42]

using < to compare unrelated pointers give unspecified result according to standard.
std::less should be consistent (even if I don't know any implementation of std::less which is different than lhs < rhs).

[attilaszenczi]

Note: The issue is about the case
using KeyT = gsl::not_null<T>;
std::map<KeyT, ValueT>

[hsutter]

Editors call: Yes, in not_null where we do t < t, we should do std::less(t, t).