Caching where-is-internal (was: (icomplete-vertical-mode): Add support for affixations and, annotations)

[Stefan Monnier]

> That is of course correct that it is better to add some internal
> pre-computation. In the case of Marginalia we also have an extra caching
> for the keybindings, since the `where-is-internal` function is
> relatively slow. This matters when computing the keybinding annotations
> on many candidates.

`where-is-internal` is supposed to do its own caching already (I added
that around the time of Emacs-21 because it was making construction of
the menubar too slow).  So if you see problems on that side, please
report them.


        Stefan