Redesign completion style definition mechanism

Redesign completion style definition mechanism

* lisp/minibuffer.el (completion-styles-alist): Don't define flex
here.
(completion-styles-try-completion)
(completion-styles-all-completions): New generics.
(completion--nth-completion): Use them.  Return a cons of
completions and metadata.
(completion-all-completions): Adjust metadata here.
(completion--flex-adjust-metadata): Return adjusted metadata
entries.
(completion-styles-try-completion flex)
(completion-styles-all-completions flex): Implement.