Hash-cons abstract values.
Hash-cons abstract values.

This slows down the test suite by about 50%, but reduces memory use of
abstract analyses by 20-30x and speeds up code generation by as much
as 50x on the benchmark examples (why??).  Oddly, this is not enough
to permit any additional benchmarks to compile.

axch/dysvunctional-language,axch/dysvunctional-language,axch/dysvunctional-language,axch/dysvunctional-language