UI improvements for structural editor (#32)

* Use std::fmt for centering, add labels for halves

This makes everything cleaner! With some algebra and code™, it's
possible to use `format!` for centering text.

This commit also adds labels to the pattern and replacement half, just
being centered boxes `| Pattern |` and `| Replacement |` in the usually
bland `--------`(...) line after the status.

* Fix rule index (off-by-one errors go brrr)

* Switch from termion to crossterm

Squash of:
- Switch from termion to crossterm (@brightly-salty)
- Abstract away a view for easier layouting (@brightly-salty)
- Add lockfile (@ThePuzzlemaker)
- Undo some AST/parser changes (@ThePuzzlemaker)

Co-authored-by: ThePuzzlemaker <tpzker@thepuzzlemaker.info>

* Fix code for new AST, make pretty_rule use &[Term] instead of Vec<Term>

* Rebase and fix

* Add failing interactive mode

* Improve interactive rewriter

* Fix prelude + mlatu.pl

* Add CLI + standardize keybindings

Co-authored-by: Caden Haustein <code@brightlysalty.33mail.com>

.gitignore

@@ -1,8 +1,6 @@
 debug/
 target/
 
-Cargo.lock
-
 **/*.rs.bk
 
 .pdb