Many common algorithms take a callback for comparing items - effectively
giving the items a user defined order.

For example, the standard library qsort() and bsearch() routines take such
a callback.  The ccan/avl module takes an identical one.  The ccan/asort
and ccan/asearch modules use a different variant: their callback takes an
additional context parameter, and is also typed via use of macros and
typesafe_cb.

This module provides helper types and macros for easily declaring any of
the common variants on comparison functions: the 2-parameter untyped form
(as used by qsort), the 3-parameter untyped form (used by the asort back
end) and the 3-parameter typed form (used by the asort front end).  It
provides a wrapper macro for doing the typesafe_cb conversion from
3-parameter typed to 3-parameter untyped.

It also provides a container struct to describe both a comparison callback
and a context value as a single structure.  This also comes in both
untyped and typed variants.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>