Update vim_diff.txt with :lmap differences, update documentation on
'keymap', and add tests.
The tests added are to demonstrate the behaviour specified in the
documentation of :loadkeymap.
This means that the major way that :lmap mappings are applied works as
one would expect with macros.
This also means that having a translation with 'keymap' does not
preclude using mappings in insert mode with :imap.
There is some behaviour that we keep with the recent changes, and some
behaviour that we change.
Instetad of having one failing test covering all behaviour, we split
the test into two.