Please test the ekr-vim branch

[Edward K. Ream]

This branch completes #1757, which fixes bugs in both vim mode and vim emulation.

- vim emulation: @keys Vim bindings
- vim mode: @bool vim-mode

Leo 6.3 broke vim emulation and put at least one crasher in vim mode.

The bug fixes are the simplest thing that could possibly work. #1757 discusses what I did in detail, including some subtle (benign) consequences.

Another highlight of the ekr-vim branch is a vim-like box cursor in command mode. See #1768.

I have been eating my own dog food for several days. Please report any problems immediately. I plan to merge ekr-vim into devel next Friday, December 11.

Edward