R1:1b192f746a67

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…