compiler/natives/src/reflect: Implement safe Swapper.
compiler/natives/src/reflect: Implement safe Swapper.

Based on go4 version, with fast path from 1.8 standard library.

Fixes TestSwapper.

gopherjs/gopherjs,gopherjs/gopherjs,albrow/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs,albrow/gopherjs,albrow/gopherjs,gopherjs/gopherjs