splitOn for Data.ByteString/Data.ByteString.Lazy

As per http://stackoverflow.com/questions/1398322/split-bytestring-on-a-bytestring-instead-of-a-word8-or-char this is a commonly requested piece of functionality. Here is one possible implementation for Data.ByteString:

```
splitOn :: Str -> Str -> [Str]
splitOn needle haystack0 = go haystack0
where
l = length needle
breaker = breakSubstring needle -- save preprocessing
go s = let (pre, post) = breaker s
in pre : if null post
then []
else go (drop l post)
```

However, for lazy bytestrings, we will need an implementation of breakSubstring for them.