Do a lot of changes

- Add ty to dev deps;
- Add some tests covering previously uncovered cases;
- Fix some bugs;
- Introduce argfield for nicer argparse interface for things that can't
  be expressed via the type system;
- Fix line length.

pyproject.toml

@@ -25,7 +25,7 @@ requires = [
 build-backend = "uv_build"
 
 [tool.ruff]
-line-length = 120
+line-length = 79
 
 [tool.ruff.lint]
 select = ["F", "B", "I"]
@@ -34,4 +34,5 @@ fixable = ["I"]
 [dependency-groups]
 dev = [
     "ruff>=0.15",
+    "ty>=0.0.27",
 ]