Add Dict filtering functions

Adds `Dict.filter`, `Dict.partition`, and a `WithKey` variant for each.

Note - this currently doesn't work. I tried to test creating a new elm module which uses these functions:
```
$ ./dist/build/elm/elm -r data/elm-runtime.js TestDict.elm 
[1 of 1] Compiling TestDict            ( TestDict.elm )
Error on line 7, column 19 to 30:
Could not find variable 'Dict.filter'.
```

Looking at `data/elm-runtime.js`, the functions are all there and in the export object - is there some other place that needs to be updated?