Add some debug support code to try to catch future mistakes in the area of

input functions that include garbage bytes in their results.  Provide a
compile-time option RANDOMIZE_ALLOCATED_MEMORY to make palloc fill returned
blocks with variable contents.  This option also makes the parser perform
conversions of literal constants twice and compare the results, emitting a
WARNING if they don't match.  (This is the code I used to catch the input
function bugs fixed in the previous commit.)  For the moment, I've set it
to be activated automatically by --enable-cassert.