Move the ABC pin on to the bounded ISOP table

[TrevorHansen]

Follows #1242, which pinned 1b977a4d5. This moves on to e19e80d78, adding stp/abc#20: the ISOP cover table that pin brought in is now bounded at 1024 entries, about 200 KB.

It needed bounding. The table inserts on every miss and never evicts, so its size was limited only by two entries per cone — O(AIG) — and at roughly 160 bytes an entry a twelve-million-gate blast could in principle have reached a gigabyte of table while getting nothing back, since in that regime almost every lookup is a miss. Real circuits are nowhere near: four functions answer all 146,304 cover requests of a 293k-AND multiplier, and the most diverse input measured presents 166. But that was a property of the circuits rather than of the code.

The bound costs nothing measurable. Replaying the request streams against a bound of 1024, an LRU of 1024 and a direct-mapped 1024 all reproduce the unbounded table's hit rate to three decimal places, so the policy is the simplest one that bounds it: past the bound a stored function is still replayed, one that is not is computed and not stored.

Verified against an ABC built at exactly e19e80d78 and reached through -DABC_DIR, since a plain configure resolves ABC from whatever libabc-pic.a another build tree installed first and would not test the pin at all: build clean, 357 of 357 tests passing.