add is_keyer macro

[Andersama]

Adds a macro to try to detect a key-like structure.

TODO: key-like structures are any integral, bitset, boolean, float, double, tuple of keys or array of keys.

Also changes is_comparer to take unsigned integers.

[lerno]

What should is_keyer do?

[lerno]

Added as @is_cmp_key_fn in dev