Change map from CTRL-; to CTRL-S

Windows cannot see CTRL-;, but Ubuntu can. Try to find a new and useful
mnemonic. Needs to be something you can hit repeatedly (so multi-segment
maps are no good unless it's the same thing repeated).

Insert mode mappings are hard.

CTRL-S is used by surround, but CTRL-g s is also available and I think
endoscope's usage pattern is more useful.

CTRL-S is also used by terminals for flow control. That's not useful so
include instructions for turning it off.