Remove Field.from_options to reduce memory usage

[rmosolgo]

I extracted this from #5491 because it's where the memory savings come from:

  $ be rake bench:profile_boot 
  # ...
- Total allocated: 8242456 bytes (51658 objects)
+ Total allocated: 7774256 bytes (46977 objects)

(I think the savings comes from Hashes allocated during the intermediary method, so savings for a given app will depend on how many field(...) calls there are.)