Tighten up effect types on modifyScope and modifyThis

Fix #19.  Since these run immediately, there is no need to expose the
unsafeInterleaveEff.
Merge pull request #21 from dylex/master

Tighten up effect types on modifyScope and modifyThis
Restrict Promise.then' not to change failure type

Fix #17
Merge pull request #22 from dylex/master

Restrict Promise.then' not to change failure type
Switch DOM.Node and DOM.Event to simple-dom
Move DOM.Types and some http to simple-dom

Use many HTTP-related types now present in simple-dom.