Improve sort by source code so that it uses natural order

[cdparra]

See https://stackoverflow.com/questions/9173558/postgresql-order-by-issue-natural-sort
or https://pgxn.org/dist/pg_natural_sort_order/

Basically, we need the following:
P1, P4, P3, P1a, P4a, P1b

to be sorted as follows:
P1, P1a, P1b, P3, P4, P4a,

Currently it sorts like this:
P1, P3, P4, P1a, P1b, P4a