WIP: Implement mark functionality

[xelxebar]

Marks are reified as underscore-prefixed variables due to their better affordances than direct strings, i.e. we can perform direct arithmetic on mark variables etc. Note, also, that updates to the document will auto-update variables as necessary. Moreover, marks are document-specific, so switching between documents with e will load and unload mark variables for you.

Note: Needs integration with undo.

The above behavior hasn't been sanity-checked against ed, however. Thoughts?

[arcfide]

Note sure how I feel about this one. I'll have to do some thinking about the design. I agree that the marks situation needs to be improved, but I wonder what the best way to do this is.