[tint] Implement some lexer optimizations

Two specific optimizations are added in this CL:
1. Cache'ing the current line state instead of fetching it with every
   call to line(), since it only changes on construction and when
   advance_line() is called.
2. Changing the if-elif chain in try_punctiation in a switch-case, so
   that instead of iterating through all 30-ish cases the code can
   just jump straight to the correct case. (This cannot be trivially
   done for parse_keyword, since it works on full strings instead of
   single chars.)

This improves runtime for the reference test, relative to baseline
another 10% to 20%.

Fixes: 563083232
Change-Id: I6b2b03e912f04739796a843500932d0ea977093e
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/344375
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Auto-Submit: Ryan Harrison <rharrison@chromium.org>
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
Reviewed-by: dan sinclair <dsinclair@chromium.org>