Use a lookup table for units in pg_size_pretty and pg_size_bytes

Use a lookup table for units in pg_size_pretty and pg_size_bytes

We've grown 2 versions of pg_size_pretty over the years, one for BIGINT
and one for NUMERIC.  Both should output the same, but keeping them in
sync is harder than needed due to neither function sharing a source of
truth about which units to use and how to transition to the next largest
unit.

Here we add a static array which defines the units that we recognize and
have both pg_size_pretty and pg_size_pretty_numeric use it.  This will
make adding any units in the future a very simple task.

The table contains all information required to allow us to also modify
pg_size_bytes to use the lookup table, so adjust that too.

There are no behavioral changes here.

Author: David Rowley
Reviewed-by: Dean Rasheed, Tom Lane, David Christensen
Discussion: https://postgr.es/m/CAApHDvru1F7qsEVL-iOHeezJ+5WVxXnyD_Jo9nht+Eh85ekK-Q@mail.gmail.com