Add support for additional rules/connectives

[Sintrastes]

There are a lot of different approaches to categorial grammar, and not necessarily a fixed set of connectives or rules.

To allow for experimentation with different approaches, we should make Montague as extensible as possible, potentially using an approach like tagless final encodings.

For instance, we could allow for both non-associative (and as an extension) associative connectives as an extension.