WIP for webmachinelearning#678

- Introduce a "cast" definition that takes a number and a type, and returns the number cast to that type.

- Invoke cast during MLOperand and MLActivation creation.

TODO:

- Passing restrictions
  - Floating point - allow Infinities/NaNs or not?
  - Integer - throw or clamp if out of range?
- Simplify supported restrictions
- resample2d sizes option - is this part of the op data or not?