* Add more rigorous type inference and encapsulate the type inferece code in its own file (ti.py).
Add constraints accumulation during type inference, to represent constraints that cannot be expressed
using bijective derivation functions between typevars.
Add testing for new type inference code.
* Additional annotations to appease mypy
The List and Dict types are no longer implicitly available. They must be
imported from typing.
Type annotations must appear before the doc comment in a function. Also
fix type errors in these functions that weren't detected before.
Each instruction used in a pattern has constraints on the types of its
operands. These constraints are expressed as symbolic type variables.
Compute type variables for each variable used in a transformation
pattern. Some are free type variables, and some are derived from the
free type variables.
The type variables associated with variables can be used for computing
the result types of replacement instructions that don't support simple
forward type inference from their inputs.
The type sets computed by this patch are conservatively too large, so
they can't yet be used to type check patterns.
