Routine usage information schema tables

Several information schema views track dependencies between 
functions/procedures and objects used by them.  These had not been
implemented so far because PostgreSQL doesn't track objects used in a
function body.  However, formally, these also show dependencies used
in parameter default expressions, which PostgreSQL does support and
track.  So for the sake of completeness, we might as well add these.
If dependency tracking for function bodies is ever implemented, these
views will automatically work correctly.

I developed this as part of the patch "SQL-standard function body", 
where it would become more useful, but I'm sending it now separately to 
not bloat the other patch further.

[0]: 
https://www.postgresql.org/message-id/flat/1c11f1eb-f00c-43b7-799d-2d44132c02d7@2ndquadrant.com