Add CONSTANT block support for modcc (#825)

Addresses #824.

* Add modcc support for `CONSTANT` blocks in NMODL, subject to the following constraints:
    - Any identifier declared in the `CONSTANT` block may only be used after that declaration (including within the `CONSTANT` block itself).
    - Units in the `CONSTANT` block are parsed but not processed.
    - Values assigned to identifiers inside the `CONSTANT` block may only be signed numeric literals, or an already declared constant, possibly preceded by unary minus.