Speed up plpgsql function startup by doing fewer pallocs.

 Speed up plpgsql function startup by doing fewer pallocs.
 
 Previously, copy_plpgsql_datum did a separate palloc for each variable
 needing instance-local storage.  In simple benchmarks this made for a
 noticeable fraction of the total runtime.  Improve it by precalculating
 the space needed for all of a function's variables and doing just one
 palloc for all of them.
 
 In passing, remove PLPGSQL_DTYPE_EXPR from the list of plpgsql "datum"
 types, since in fact it has nothing in common with the others, and there
 is noplace that needs to discriminate on the basis of dtype between an
 expression and any type of datum.  And add comments clarifying which
 datum struct fields are generic and which aren't.
 
 Tom Lane, reviewed by Pavel Stehule
 
 Discussion: https://postgr.es/m/11986.1514407114@sss.pgh.pa.us