This adds macros to implement (and/or declare) type-safe wrapper functions

around the callbacks required in the LHASH code for the "doall" functions.

Also - fix the evil function pointer casting in the two lh_doall functions
by deferring to a static utility function. Previously lh_doall() was
invoking lh_doall_arg() by casting the callback to the 2-parameter
prototype and passing in a NULL argument. This appears to have been working
thus far but it's not a hot idea. If the extra level of indirection becomes
a performance hit, we can just provide two virtually identical
implementations for each variant later on.