This allows the removal of its O(n) .get_field_by_name method and many
other awkward access patterns.
While fields were initially stored in a list to preserve the initial
model definiton field ordering the auto-detector doesn't take field
ordering into account and no operations exists to reorder fields of a
model.
This makes the preservation of the field ordering completely superflous
because field reorganization after the creation of the model state
wouldn't be taken into account.
This moves all the field referencing resolution methods to shared
functions instead of duplicating efforts amongst state_forwards and
references methods.
Simon Charette
Ville Skyttä
Tim Graham