Build out the modification API

[sorear]

Current assumption is that you can "open" a database, and then get write cursors that work on a statement level of abstraction and can modify math strings, etc. We don't expose (most? nameck data might be useful) nonlocal data while the database is open (if you want that, .clone() the database first — we'll make sure that's cheap). When you're done with a batch of edits, you can close the write cursors and the nonlocal data will recalculate using the existing incremental functionality.

Make this work, and provide examples. For instance: safe statement renaming that won't mess with comments.

[digama0]

"won't mess with comments": ...except in ~ label references in comments. Same with constant renames.