Rename snappy methods

Summary: Renames sncompress to snappy_compress and snuncompress to snappy_uncompress to maintain code compatibility with PHP (when using php-snappy):

https://code.google.com/p/php-snappy/

Fixes compatibility specifically with:
https://github.com/colinmollenhour/Cm_Cache_Backend_Redis/blob/master/Cm/Cache/Backend/Redis.php#L857

Replaces old PR due to incorrect commit message

Closes https://github.com/facebook/hhvm/pull/3259

Reviewed By: ptarjan, joelm

Differential Revision: D1452331

Pulled By: svcscm