Add a type Reducer for recording extra reductions

In the chain-rule machine, we need to skip through edges whose labels
are "accepting", otherwise the time complexity will be high even for
simple grammars.  This implies that we will skip some "jumping up" in
the item derivation forest.  So we need to record these extra jumping
up, in order to jump up at a later point.

This Reducer type plays this role.  But I still need more experiments
to see if this approach works out as I intended.

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