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.

Fix leftover from _Name -> name transition in Variable.
Change a few pymbolic.product to pymbolic.flattened_product in
  ExpandMapper.
Let ExpandMapper handle constants properly.

Remove is_constant from differentiator.
Trivial fixes.

Fix subscript operator mapping.
In overloaded operators: Don't "assert is_valid_operator()",
  return NotImplemented instead.
Make sure Sum and Product do not allow invalid operands
  into the expression tree.

Flatten mapper.
Let mappers deal with objects they know nothing about in a
  structured way.
Small fixes to expand mapper.
Stringifier knows when to parenthesize (-1).
Expression sums optimized.