Implement `ExpressionEvaluator`

[marvinlanhenke]

As mentioned in #348:

We need to implement the ExpressionEvaluator, that binds the partition_spec to the 102: partition struct and evaluates it.

Python implementation (or the start of the workflow) can be found here.

[marvinlanhenke]

I've already taken a look at this; so I'd like to work on this myself in the upcoming week.