std: Rename `assert` and `assert_eq` to `fail_unless` and `fail_unless_eq`.

These macros are always enabled so there's no particular need to fly in
the face of established convention where assertions are removed in
production code.

Closes #12049.
std: add `debug_assert!` and `debug_assert_eq!`, removed for --cfg ndebug.
Rename uses of `assert!` to `fail_unless!`.
Rename uses of `assert_eq!` to `fail_unless_eq!`.