add arena evaluator as symbolic alternative, test

[maxdeliso]

Adds an alternative evaluator which uses a hash-consing arena which guarantees exactly one representation per tree, using pseudorandom integer distribution and a bucketing scheme.

               ┌────────────  “arena”  ────────────┐
               │  parallel columns (contiguous)    │
 index (id)    │ kind  sym  left  right  hash32    │     bucket table
───────────────┼──────────────────────────────── ──┤   (hash → head id)
     0  (S)    │  0     1     –      –       1     │    ┌─┐
     1  (K)    │  0     2     –      –       2     │ h0 │3│──►2──►…  (nextIdx list)
     2  (I)    │  0     3     –      –       3     │    └─┘
     3         │  1     –     0      1     0xA7…   │ h1 ─┬──►4
     4         │  1     –     3      2     0x5C…   │ h2 ─┴──►…
     …         │  …                                │
               └───────────────────────────────────┘

see also: https://www.cs.tufts.edu/~nr/cs257/archive/henry-baker/cons-lazy-alloc.pdf