moved Stream instance for lists (Text.Parsec.String to Text.Parsec.Prim)

I think Text.Parsec.Prim is a more natural place for this, since it's now adjacent to the typeclass definition  (and as such, Text.Parsec.String doesn't use orphan instances now, either).

I had some trouble finding this instance while using parsec on a custom token type.  Since I didn't want to deal with conflicting names, I didn't want to import Text.Parsec or Text.Parsec.String directly. (although, I did eventually import Text.Parsec.String (), which works).

I think this change will make it easier for others to find the instance.