Cleaner default resolver, cache FieldDefMap Get, preallocate

[ajackson-cpi]

Returning 26k rows for me was taking 8 seconds of post-Resolve() processing. I reduced that to 305ms by:

• Returning the cached FieldDefMap (7 seconds)
• Preallocating result map/array structures (5ms or so)
• DefaultResolver logically identical, but far more efficient: uses reflect.FieldByName instead of checking in a loop. Allocates fewer variables.

[bbuck]

It might be worth building a lazy cache for different types as well, so we don't have to keep looping fields all the time. Maybe not as part of this PR (nice work) but just a general remark.

[edpark11]

Thanks for writing this! An (admittedly large) graphql list query was taking 10s.
This piece in particular (returning the cached field definition map) resulted in a 8x speedup. Possible to accept that piece of the patch? The default resolver patch currently breaks against the master branch but the cached field definition (just 3 lines) works fine.

func (gt *Object) Fields() FieldDefinitionMap {                                      
  if len(gt.fields) != 0 {
    return gt.fields
  }

[ajackson-cpi]

It looks like the latest master has brought equivalent changes. My performance evaluation shows it quite similar.

[chris-ramon]

Thank you for given latest changes on master a try @ajackson-cpi! there was a similar optimization introduced by #265, a couple of opt. missing from this PR:

• 7efae5e#diff-58bccd1b2cfaf06d4d4ac893e4712c88L281
• 7efae5e#diff-58bccd1b2cfaf06d4d4ac893e4712c88L793

We might want to add those via a new PR.