More string searching algorithms

[make-github-pseudonymous-again]

See:

• https://blog.burntsushi.net/ripgrep/#literal-optimizations.
• http://0x80.pl/notesen/2016-11-28-simd-strfind.html
• http://www-igm.univ-mlv.fr/~lecroq/string/

Implement at https://github.com/string-searching.