Embedded runtime as string lits

This allows elm-compiler to be used as a Haskell library, going from Elm source to JS code without ever going into IO.

It avoids trickiness of having  to have core.js and debug.js installed somewhere on the user's system.

It's not super high priority. I can work around it, but I'll basically just end up embedding the current versions of the files in my elm-build-lib library, and manually updating them with each release of Elm. So having this would be more convenient.

Cons: introduces dependency on file-embed and template Haskell. Possibly increases size of executable by including the values as strings.