need a function to extract list items from pg_node_tree

In order to implement the PARAMETER_DEFAULT column in the information
schema I need a way to get the expressions out of the proargdefaults
column. pg_get_expr(proargdefaults, 0) gives me all expressions
comma-separated, but I need them individually. I think a function like
pg_get_list_nth (to keep consistent with the internal list API) could
work. Alternatively, a list-to-array function might do the trick. Are
there are any other potential uses cases that should be considered here?
AFAICT, indexprs is the only other system catalog column that contains
an expression list.