Permit multiple threads to share a common Environment

The `Environment` class saves a copy of the most recently-created `Runnable` program instance.

This can make it awkward to have an `Environment` shared by multiple threads creating thread-specific ``Runnable`` objects to do concurrent evaluations of CEL expressions.

Remove the `self.runnable` attribute and return the `Runnable` instance from `Environment.program()`

See issue #38