Expose the hotswap function for easier maintainability

We need another option for elm-debugger. We shouldn't have two copies of the hot swap function when they do the same thing in both the reactor and on debug.elm-lang.org.

We only need to expose this internal function when we're on debug.elm-lang.org.

After this, I can't think of anything else stopped me from finished the import of the new debugger into the debug.elm-lang.org repo.