Bug 23893: Implement Koha::Object->from_api_mapping

Bug 23893: Implement Koha::Object->from_api_mapping

This patch implements the from_api_mapping method, that calculates (and
caches) the reserve mapping from the to_api_mapping method.

A generic to_api_mapping is added to simplify things a bit in the generic
to_api method and so the mappings are reusable in the way they are in
from_api_mapping.

To test:
1. Apply this patches
2. Run:
  $ kshell
 k$ prove t/db_dependent/Koha/Object.t
=> SUCCESS: Tests pass!
3. Sign off :-D

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>