Add Dict.filter
Add Dict.{filter(WithKey),partition(WithKey)}

Not quite working - need to work out why the compiler thinks these new
functions are not in scope.
Add tests for new Dict functions
Add Set.{filter,partition} with tests
Fix Set.{filter,partition}
Remove Dict.{filter,partition}WithKey
document Dict.{filter,partition}
Merge pull request #586 from hdgarrood/dict-filter

Add Dict and Set filtering functions