basic-bytestring wrapper does not work with left contexts

The `basic-bytestring` wrapper does not work correctly with left contexts when provided with characters which are encoded as multiple bytes in UTF-8.

The following program produces `True,False` while I expect it to produce `True,True`.

```
{
{-# LANGUAGE OverloadedStrings #-}
module Main where
import qualified Data.ByteString.Lazy.Char8 as B
}

%wrapper "basic-bytestring"

tokens :-
a^b {const True}
a {const True}
∃^∀ {const True}
∃ {const True}
. {const False}

{
main::IO ()
main = do
print . and . alexScanTokens $ "ab"
print . and . alexScanTokens $ "∃∀"
}
```

I think this is due to [`alexGetByte`](https://github.com/simonmar/alex/blob/master/templates/wrappers.hs#L104) for this wrapper remembering the last byte rather than the last _character_.

Since converting input bytes to characters puts unnecessary costs on the users of this wrapper maybe we should just not implement left contexts in this case?