Revert "The fix for #32 implied an API change, so document it"

This reverts commit 4f74772e8ca3916b8d7ccdd729e6245999e1fef8.
Revert "Fix the token length for ByteString wrappers (#32)"

This reverts commit a325ddefd46923c678a05277fbc91e7deec73e4d.
On second thoughts, fix #32 without an API change.

The length returned in AlexReturn is really bogus, we should be moving
towards clients managing the token length themselves as part of
AlexInput.  GHC itself has always done this, and now the ByteString
wrappers do it too.  We should make the other wrappers keep track of
their own token length, and then we could remove the length field of
AlexToken/AlexSkip.  Maybe we should make these constructors into
records first.