In particular, there was a typo that kept library values from being
updated, and there was missing logic to fetch the various field names
from the library parts (and the change-to part).
Also implements some performance gains by desisting from copying
LIB_FIELDs around every time we want to look at them.
Fixes https://gitlab.com/kicad/code/kicad/issues/6733
Fixes https://gitlab.com/kicad/code/kicad/issues/6749