Declare assorted array functions using anycompatible not anyelement.

Declare assorted array functions using anycompatible not anyelement.

Convert array_append, array_prepend, array_cat, array_position,
array_positions, array_remove, array_replace, and width_bucket
to use anycompatiblearray.  This is a simple extension of commit
5c292e6b9 to hit some other places where there's a pretty obvious
gain in usability from doing so.

Ideally we'd also modify other functions taking multiple old-style
polymorphic arguments.  But most of the remainder are tied into one
or more operator classes, making any such change a much larger can of
worms than I desire to open right now.

Discussion: https://postgr.es/m/77675130-89da-dab1-51dd-492c93dcf5d1@postgresfriends.org