Add variable assignment

[eliot-akira]

This adds a variable assignment operator = as per #65.

It passes all existing tests, as well as a few more I added to confirm basic functioning.

Some of points you raised in the issue are not addressed:

• concept of statements with ; - I figured this feature can be another pull request
• expression evaluation is side-effect free - by design, variable assignment mutates the given variables object
• support should be opt-in - I wasn't sure the best way to implement this; I suppose similar to the in operator?

[silentmatt]

Thanks for contributing this! I have a couple small changes to allow chaining (e.g. x = y = z+1), but for the most part this looks good.