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.

uv.lock

@@ -10,12 +10,16 @@ source = { editable = "." }
 [package.dev-dependencies]
 dev = [
     { name = "ruff" },
+    { name = "ty" },
 ]
 
 [package.metadata]
 
 [package.metadata.requires-dev]
-dev = [{ name = "ruff", specifier = ">=0.15" }]
+dev = [
+    { name = "ruff", specifier = ">=0.15" },
+    { name = "ty", specifier = ">=0.0.27" },
+]
 
 [[package]]
 name = "ruff"
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 ]
+
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