Throw error for unsatisfiable transition constraint due to base vs ext field

[grjte]

This is an example of a problematic constraint:

enf b = a + $rand[0]

Context from Bobbin:
This probably should be invalid as this constraint should never be satisfied: the right hand side is an extension field element, while the left hand side is a base field element. If the value in $rand[0] is truly random, these will never be equal.

We'll need to think about a general rule which can describe this.

Originally posted by @bobbinth in #36 (comment)