Rename @/folly:folly-sse42 to @/folly:checksum
Rename @/folly:folly-sse42 to @/folly:checksum

Summary: To better reflect what it actually is.

Reviewed By: yfeldblum

Differential Revision: D5186774

fbshipit-source-id: c13b5f6c93c3764a28510d825faec0fb4232d1f2

ldemailly/wdt,luluandleilei/wdt,luluandleilei/wdt,ldemailly/wdt,luluandleilei/wdt,ldemailly/wdt,ldemailly/wdt,luluandleilei/wdt