The stray-{ else branch in tokenize_fast had a nested while loop that could
exit via its condition (when next_open >= len) without advancing pos.
The outer while pos < len loop would then find the same { again forever.
Reproduced by: Liquid::Template.parse('a{') -- hangs indefinitely.
Replaces the nested scan loop with two String#byteindex calls to find the
next '{%' and '{{' directly, then takes the minimum. Always O(n), eliminates
the nested loop entirely, and impossible to leave pos stranded.
Adds regression tests covering the three inputs that previously hung
plus adjacent stray-brace cases.
Chris Pak
d9c42fd2eb