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

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.