Grammar: Add 'c' to compass point values

[ferdnyc]

Fixes #307

The grammar also lists _ in the list of values, but I can't tell if that's a literal underscore or some sort of placeholder, because a note just below also says this:

Note also that the allowed compass point values are not keywords, so these strings can be used elsewhere as ordinary identifiers and, conversely, the parser will actually accept any identifier.

Edit: Added some tests, as well. Confirmed that they pass on the branch, but fail if I manually revert 56059ae.