Improve performance when constructing custom fields for objects

Summary:
Ref T11404. This improves things by about 10%:

• Use PhutilClassMapQuery, which has slightly better caching.
• Do a little less work to generate pretty error messages.
• Make the "disabled" code a little faster (and sort of clearer, too?) by doing less fancy stuff.

These are pretty minor adjustments and not the sort of optimizations I'd make normally, but this code gets called ~100x (once per revision) and generates ~10 fields normally, so even small savings can amount to something.

(I also want to try to make arc faster in the next update, and improving Conduit performance helps with that.)

Test Plan: Ran differential.revision.search, saw cost drop from ~195ms to ~170ms locally.

Reviewers: yelirekim, chad

Reviewed By: chad

Maniphest Tasks: T11404

Differential Revision: https://secure.phabricator.com/D16355