folly::Function: fix swap function and put in correct namespace

folly::Function: fix swap function and put in correct namespace

Summary:The swap function belongs in the same namespace as Function: folly.
Also, to avoid amibiguities with a generic swap function in
<algorithm>, we need two variants: one for identical types of
folly::Function, and one for folly::Functions with different
configurations.

Reviewed By: ericniebler

Differential Revision: D3106429

fb-gh-sync-id: 11b04e9bc709d52016ac94c078278410f5ee43c6
fbshipit-source-id: 11b04e9bc709d52016ac94c078278410f5ee43c6