Add a SymbolKindTable to DAGCode, make infer_kinds() a transformation

This adds a new attribute called *sym_kind_table* to DAGCode which is
a SymbolKindTable. The function *infer_kinds* is changed to be a
transformation that returns a modified DAGCode object with all symbol
kinds inferred.

Since DAGCode is immutable, SymbolKindTable is changed to be immutable
as well. The interface is changed to make SymbolKindTable free of side
effects. To maintain efficiency, the underlying dicts are replaced
with persistent dictionaries provided by pyrsistent.

This change also makes it an error when unification fails in
SymbolKindTable.set()

Closes #35 by providing an interface to supply kind information when
kind inference won't work