Split array_push into separate array_append and array_prepend functions.

 Split array_push into separate array_append and array_prepend functions.
 
 There wasn't any good reason for a single C function to implement both
 these SQL functions: it saved very little code overall, and it required
 significant pushups to re-determine at runtime which case applied.  Redoing
 it as two functions ends up with just slightly more lines of code, but it's
 simpler to understand, and faster too because we need not repeat syscache
 lookups on every call.
 
 An important side benefit is that this eliminates the only case in which
 different aliases of the same C function had both anyarray and anyelement
 arguments at the same position, which would almost always be a mistake.
 The opr_sanity regression test will now notice such mistakes since there's
 no longer a valid case where it happens.