After studying the list of exposed functions for awhile, it seemed to me that we should also expose exec_assign_value. The new pointers allow a plugin to compute a value in Datum+isnull format, but then it can't do much of anything with it: exec_assign_expr is a completely inconvenient API if what you want to do is put a specific Datum value into a variable. Adding exec_assign_value provides "store" and "fetch" APIs that are more or less inverses, which should be easier to work with.
