Use binary search for synonyms, fixes #31

[doozan]

This replaces the SynFile::load and SynFile::search with logic borrowed from WordListIndex, resulting in dramatic speed improvement on dictionaries with large synonyms.

Before:

$ time ./sdcv -2 dicts/ hola
Found 1 items, similar to hola.
-->Wiktionary (es-en)
-->hola

hola (interj)
1. hello, hi

real    0m8.084s
user    0m7.979s
sys     0m0.090s

After

Found 1 items, similar to hola.
-->Wiktionary (es-en)
-->hola

hola (interj)
1. hello, hi

real    0m0.095s
user    0m0.039s
sys     0m0.042s

[Dushistov]

Thanks