Formulas should not silently swallow literals during materialization.

[matthewwardrop]

Currently in formulaic during materialization, bare literals will be silently dropped. In some sense, this makes sense, but given that these are extraneous, an error should be raised.

For example:

import pandas
import formulaic
formulaic.model_matrix('a + b + 50', pandas.DataFrame({'a': [1,2,3], 'b': ['a', 'b', 'c']}))