folly/ApplyTuple.h: fix const-correctness & other issues, simplify

folly/ApplyTuple.h: fix const-correctness & other issues, simplify

Summary:The existing implementation of folly::applyTuple does not support
mutable callables (such as mutable lambdas and other functor objects
that only implement non-const operator()).

This commit adds a few more unit tests and changes the implementation
so that new and existing tests pass.

Reviewed By: yfeldblum

Differential Revision: D2942622

fb-gh-sync-id: 82478f290e9fd2020358ff79ef0a6bcf8a43738c
shipit-source-id: 82478f290e9fd2020358ff79ef0a6bcf8a43738c