Fix error in `s:unlet_for` when using `Runtime` on an empty buffer
Add MapHelp command

MapHelp makes it easier to lookup what a command does. Most of the time,
it returns whatever you've mapped the input key sequence to, but
if it's built-in it shows vimdoc for the map (instead of an error like
:map).

Useful whenever you know you use something a lot, but can't remember
where it comes from.

Also includes a bang variant to use :Verbose instead of verbose.
Handle maps containing Ctrl

Ctrl is input differently for maps and help. Users always use map-style
input, so when we're looking in help we must convert to help-style.