Allow completion styles to adjust completion metadata

This commit re-does the now-reverted commit with the same title.  That
version relied on generic functions, which cannot be used yet in files
such as lisp/minibuffer.el.  This version uses a symbol property
completion--adjust-metadata instead.

The new facility allows completion styles to have a say in metadata
properties such as cycle-sort-function and display-sort-function.
This is especially useful for completion styles such as 'flex', which
generally produce many matches, including some potentially "obscure"
ones.  The default sorting strategy would often bubble the latter to
the top of the list.

The sorting function for 'flex' considers pre-computed matching scores
and is thus much better than the default for this particular style.

Additionally, it overrides the completion table's cycle-sort-function
or display-sort-function properties if they exist, although it still
uses them to pre-sort the result, so that they are still relevant for
resolving ties.

* lisp/minibuffer.el (completion--nth-completion)
(completion--flex-adjust-metadata): New helper.
(flex): Put completion--adjust-metadata property.

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