Include the ThreeIterator permutation checks

This adds the 3-character Token validation checks. It checks every
permutation of characters in the range 0..=0x7F for length one, two, and
three. This results in 4,227,328 checks which on my system takes about
22 seconds. On CI it will be much longer, but should be tolerable on a
nightly job.

Author: ehuss

.github/workflows/daily-grammar-check.yml

@@ -40,6 +40,7 @@ jobs:
       run: |
         cargo run --release -p grammar-check -- lex-compare --path rust
         cargo run --release -p grammar-check -- lex-compare --permute Token --tool rustc_parse
+        cargo run --release -p grammar-check -- lex-compare --permute three
 
     - name: Check for existing open issues
       if: steps.grammar-check.outcome == 'failure'