Cache typlens of a SQL function's input arguments.

Cache typlens of a SQL function's input arguments.

This gets rid of repetitive get_typlen calls in postquel_sub_params,
which show up as costing a few percent of the runtime in simple test
cases (more with more parameters).

In combination with the preceding patches, this gets us most of the
way back down to the amount of per-call overhead that functions.c
had before commit 0dca5d68d.  There are some more things that could
be done, but this seems like an okay place to stop for v18.