Move `reduce` taint flag out of `e.target()`

[SSoelvsten]

We ought to move the taint flag from the e.target().is_flagged() into a separate e.flags[REDUCE_FLAG::CUT_TAINT]. This provides the basis for us to add many of the extensions.

• For #433, we need to free the current flag to be used for the complementation flag.
• #434 requires forwarding the hash of the children.
• #435 requires knowing the variable describing the node and describing its children.

Tasks

• [ ] Define reduce_arc type to publicly inherit from arc
• [ ] Add private uint8_t flags variable and a flag(enum) function to access it. Add set_flag(enum) to set the flag.
• [ ] Replace usage of the flagging on target with using the flag.

[SSoelvsten]

See #479 for a reference implementation. But, it probably needs to be fully reimplemented from scratch to deal with the merge conflicts.