Don't id-map unsupported expressions by default.

Change stringifier constant handling to custom function.
Allow numpy arrays as constants.
Change zero detection to accomodate numpy arrays.

Mapper: Variable is an AlgebraicLeaf.
Fix NonrecursiveIdentityMapper.

make_variable() only converts non-Expressions now.

- Error out if expression is not parsed completely.
- Same-precedence operators get parsed left-to-right now,
fixing division semantics.
- allow negative exponents in sci notation.