When lazy-loading tables, copy ValueFormat

Otherwise it may be overwritten before we use it.
Minor
Remove tableStack in favor of countVars

We only needed tableStack to look back for counts.  So, just store
counts.  Gives ~8 speedup for large fonts I tested.  We are not
faster than not loading fonts lazily without this patch, so the
laziness patch combined with this doesn't have a net negative
performance impact anymore.
Make ValueRecordFactory() immutable

Gives another 3% speedup.
Use slots for OTTableReader

Slight performance gain even.