unify -> unify_seq in seq_registry
test seq_registry
add seq_registry
sorting in seteq occurs in try block
assoccomm uses op_registry instead of tuples

Previously associative commutative unification assumed that terms were
tuple-trees.  Now it consults a registry of functions to define what
terms can be.

This commit introduces a doctest error.
cull some seq_registry code
op_args in eq_assoc and assoccomm
pull build out of buildo
use build instead of (op,) + args
skip hash sensitive doctest
op_registry uses dicts instead of tuples
_as/from_tuple in unify
assoccomm uses unify _as/from_tuple
unify.py -> unification.py
rename variables.py to variable.py
remove debug code
add laziness to docs/basic.md
_as/from_tuple -> _as/from_logpy
add logify
logify works on slot classes