minmax: New module, safe min and max macros

Add a 'minmax' module with typesafe macros to compute minimum, maximum and
clamping.  Inspired by the versions used in the Linux kernel, but using
a different implementation based on __builtin_types_compatible_p() and the
build_assert module.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>