Run TemplateHaskell with sandboxing

From Luite:

> By the way, GHCJS runs Template Haskell with a JS engine now (it used to load native code) so it's not all that hard to build a variant that is safe enough for untrusted code. The standard implementation is a [small server script](https://github.com/ghcjs/ghcjs/blob/master/lib/etc/thrunner.js) that runs on node.js. GHCJS first sends the runtime system and the actual [Template Haskell runner code](https://github.com/ghcjs/ghcjs-prim/blob/master/GHCJS/Prim/TH/Eval.hs), and then incrementally all compiled splices and their dependencies. You could for example modify the script to use a [node.js sandbox](https://github.com/gf3/sandbox), if you're more adventurous you could even send it to the user's browser.