Support ^R (reverse incremental search) in dill.

^G to cancel. Canceling should probably remember your previous
history position, but doesn't.

This adds a field to ++bead, so in the absence of proper migration
support, I've simply disabled dill's nest check in ++load. It works
in this case, presumably because dill's only bead is null during
:reboot.
Merge pull request #84 from dhaffey/c-r

Support ^R (reverse incremental search) in dill.