Field method refactor for Marc transform (#200)

Why these changes are being introduced:
* These updates are required to implement the architecture described
in the following ADR: https://github.com/MITLibraries/transmogrifier/blob/main/docs/adrs/0005-field-methods.md

How this addresses that need:
* Add field methods and corresponding unit tests:
  alternate_titles, call_numbers
* Add private methods for key MARC elements:
  leader and control field '008'
* Rename 'xml' -> 'source_record'
* Update dependencies

Side effects of this change:
* None

Relevant ticket(s):
* https://mitlibraries.atlassian.net/browse/TIMX-288