util.h: add MACRO_MAP_CAT() and MACRO_MAP_CAT_N()

This is like MACRO_MAP(), but it pastes the results together into a
single token. The result is kind of a fold of the ## operator.

I wasn't able to figure out a way to implement this using any of the
existing macros, so there's some more copy/pasting of handler macros
for different numbers of arguments.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>