Instead of using a hash-table, use the equal-hash-code directly;
this lets me evict entries only when they clobber each other,
and generally keep good cache utilization.
Also, cut the cache size by a factor of 5 while still having a
slight performance improvement on the r6rs test suite benchmark.
On that same benchmark, there are 1714812 misses in the cache, but
only 3485 times is an entry in the cache clobbered
