Expanded a comment to provide more context.

This makes it more clear why it's important not to collide with ascii
values.
Moved skil_token's declaration above its comment.

Seeing the declaration first, especially the type, provides important
context.
More consistent terminology.

I used "token type" everywhere else, I should use it here too.
Some initial work on a test suite.

TODO:
 - integrate this into the buid system
 - flesh out how this is going to work in more detail.
Changed the names of token types.

We now prefix token types with `SKIL_TOKEN_`, rather than just `SKIL_`.
Otherwise, a namespace collision with other parts of the interpreter
seems probable.
Started work on a parser.

Doesn't actually function as of yet - I've mostly defined the interface
to it, as well as the AST it generates.