folly: Function: in-class init, drop (void*) casts
folly: Function: in-class init, drop (void*) casts

Summary: A few code cleanups, no functionality changes.

Reviewed By: ericniebler

Differential Revision: D3231369

fb-gh-sync-id: 1a1c8508aab6dce7279e9dfc3f3da6add5496c67
fbshipit-source-id: 1a1c8508aab6dce7279e9dfc3f3da6add5496c67

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