Add property-based tests for parser and type checker

Use `proptest` to assert that the parser and type checker never panic
on arbitrary input, and that well-formed programs drawn from a small
grammar parse and type check without errors. The regression file is
checked in so the failing cases we've already found are rerun on each
test run.

The property tests uncovered three real bugs, which are also fixed
here:

- The lexer panicked on non-ASCII whitespace because it advanced by
  one byte rather than the character's UTF-8 length.
- The lexer panicked on unrecognized non-ASCII characters for the
  same reason when constructing the "unrecognized syntax" diagnostic.
- `parse_tuple_literal_or_parentheses` hit a forward-progress
  assertion on inputs like `(,{` where the inner expression parser
  emitted a diagnostic without consuming a token. It now breaks out
  of the loop instead.

Author: claude

CHANGELOG.md

@@ -2,6 +2,21 @@
 **Goal: Better REPL experience.**
 **Goal: Offer actions when runtime errors occur.**
 
+## Parser
+
+Fixed a panic in the lexer when the input contained non-ASCII
+whitespace or unrecognized non-ASCII characters.
+
+Fixed an assertion failure in the parser when a tuple literal
+contained a token that could not start an expression (e.g. `(,{`).
+
+## Testing
+
+Added property-based tests (using `proptest`) for the parser and type
+checker. These assert that the parser and type checker never panic on
+arbitrary input, and that well-formed programs drawn from a small
+grammar parse and type check cleanly.
+
 ## Checks
 
 Added a warning when all code paths in a function return the same

Cargo.lock

@@ -47,6 +47,7 @@ assert_cmd = "2.0.14"
 escargot = "0.5"
 goldentests = "1.1.1"
 predicates = "3"
+proptest = "1.5"
 
 [profile.release]
 opt-level = "s"

Cargo.toml

@@ -0,0 +1,12 @@
+# Seeds for failure cases proptest has generated in the past. It is
+# automatically read and these particular cases re-run before any
+# novel cases are generated.
+#
+# It is recommended to check this file in to source control so that
+# everyone who runs the test benefits from these saved cases.
+cc 6e4163cef3b83d46ada9f7396d453a11a15d5db48b13c35acfcb81e705b195ca # shrinks to src = "({ let v_a = 0 0 } == 0)"
+cc 071174ad58463d80cc8a81f56ddc19058349221676b2affdfe5754af97271921 # shrinks to src = "fun v_a() { ({ let v_a = 0 0 } - 0) }"
+cc 66aae6bce59293271a3e28704a6cccba3dd46b4e2edf268d9e91189f0d999c19 # shrinks to src = "ල"
+cc 68d49a1eb4670e73cacec4ff1f8e80e43f6aaa08eb7763395d0a3f057d603d93 # shrinks to src = "0¡"
+cc 143c7d57c4fa73d032bc4a20b7eabe98c1be20f0f3e76df1cc3f9e4b9a8c1253 # shrinks to src = "\u{a0}"
+cc b1e6ee18e2705b42c49fdfa6e42c948a921eab25e9ef23682a6ac94079596205 # shrinks to src = "(,{"