Fix type coercion bugs in expr.

All tests pass now; this fixes the 2 remaining failures, including a

The structure of the code has changed a lot -- instead of having a tiny
function per operator, we have eval_op() which does common type coercion
and then evaluates the operator.  I tried writing it a couple different
ways, and this was the cleanest.

The OPS table now contains the operator string, precedence level,
signature for type coercion, and operator ID.
2 files changed