Speedup String#{ indexOf, lastIndexOf }

[MaxGraey]

This speedup indexOf matching, especially for long strings and single character.

Benchmark results:

Old - SAMPLE22_0:ok: 69.700 ms - SAMPLE22_0:eof: 49.000 ms - SAMPLE63_1:ok: 90.700 ms - SAMPLE63_1:eof: 87.200 ms - SAMPLE1k_h:ok: 345.100 ms - SAMPLE1k_h:eof: 723.800 ms New - SAMPLE22_0:ok: 40.600 ms (~1.7x) - SAMPLE22_0:eof: 39.400 ms (~1.3x) - SAMPLE63_1:ok: 64.900 ms (~1.4x) - SAMPLE63_1:eof: 54.900 ms (~1.6x) - SAMPLE1k_h:ok: 318.900 ms (~1.0x) - SAMPLE1k_h:eof: 350.800 ms (~2.0x)

Benchmark Playground: https://tinyurl.com/pjftv9pp

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