Use dupIO to avoid unwanted sharing when encoding

[adamgundry]

This introduces an optional dependency on dupIO for use when encoding, to avoid unwanted sharing as described at #318. (See #319 for an alternative approach adding a new token constructor.) I've also incorporated a fix to #317, though this doesn't seem to make a massive difference to encoding performance.

It calls dupIO before evaluating each token in the stream. Originally I tried calling dupIO only once the buffer was full, but testing the approach on the client's codebase indicated that this was not enough to avoid the issue.

Since it is unclear how much performance cost dupIO incurs in general, and it may be undesirable as a dependency, it is gated behind an off-by-default flag. (This raises the question of how to test it in CI...)