Make plpgsql treat the input collation as a polymorphism variable, so
that we cache separate plans for each input collation that's used in a
particular session, as per recent discussion.  Propagate the input
collation to all collatable input parameters.

I chose to also propagate the input collation to all declared variables of
collatable types, which is a bit more debatable but seems to be necessary
for non-astonishing behavior.  (Copying a parameter into a separate local
variable shouldn't result in a change of behavior, for example.)  There is
enough infrastructure here to support declaring a collation for each local
variable to override that default, but I thought we should wait to see what
the field demand is before adding such a feature.

In passing, remove exec_get_rec_fieldtype(), which wasn't used anywhere.

Documentation patch to follow.