Add a new `db_update()` function to update an existing database record
from a YAML hash.

Had a little trouble building up the params for the prepared SQL
statement but finally managed to set up the types correctly and include
the record ID to update. Thanks to this Stack Overflow answer by
Shepmaster (https://stackoverflow.com/users/155423/shepmaster) for
pointing me in the right direction:
https://stackoverflow.com/questions/46624591/cannot-call-rusqlites-query-because-it-expects-the-type-rusqlitetypestos/46625932#46625932

Allow passing the primary key either by value or prefixed by its column
name.

Split `db_insert()` into one function to do the YAML document recursion
and another to do the database insertion.

This will allow us to reuse the YAML document recursion function for
database `UPDATE` queries.

I'm not currently using that feature. I had added it for debugging.