Optimize startswith

[strtgbb]

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Description of Changes

Please describe here your proposed changes.

1. Optimized single-character calls to startswith. Small performance improvement.
2. Replaced startswith in fails.py transform with regex. Reduced run time of transform nice-new-fails by ~85%

before

time tfs transform nice-new-fails raw\(28\).log > /dev/null

real	0m13.005s
user	0m12.740s
sys	0m0.262s

after

time tfs transform nice-new-fails raw\(28\).log > /dev/null

real	0m1.919s
user	0m1.886s
sys	0m0.033s

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[strtgbb]

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[strtgbb]

This PR also fixes the high memory consumption of nice-new-fails although I am not sure why.

[strtgbb]

If there's any chance that result can be an empty string, then my first optimization has introduced a bug. It's not the main contributor to the performance difference, so I can revert it or put it in a wrapper that catches the possible error.