Add `eof` for Token parsers?

Something like this:

``` purescript
eof :: forall a. Parser (List a) Unit
eof =
  get >>= \(input :: List a) ->
    unless (null input) (fail "expected eof")
```

I guess it would go into `Text.Parsing.Parser.Token?

By the way, is it right that we need that type annotation on `input`? Without it, type unification fails.
