Improve handling of slow completion tables and exit functions

Improve handling of input during completion

Fix handling of non-zero completion base value

* (completion-preview-mode): Simplify mode line lighter

Only keep the ':exit-function' completion property

Avoid passing on properties such as ':predicate', whose value may be a
function that assume a certain structure of completion candidate,
which may no longer hold.

* completion-preview.el (completion-preview-show): Only keep the
':exit-function' property.

Use a trivial completion table instead of a bespoke function

Completion at point functions are encouraged to return well formed
completion table with metadata instead of a completion function.

* completion-preview.el (completion-preview-show): Simplify.
(completion-preview-insert-function):
(completion-preview--insert) Remove.
(completion-preview-insert): New function.
(completion-preview-active-mode): Adapt.

Remove 'completion-preview-default-predicate'

This variable lacks a clear use case.

* completion-preview.el (completion-preview-default-predicate): Remove.
(completion-preview-show): Adapt.

Do not 'Package-Require' Compat

Use a dedicated capf instead of rebinding 'C-M-i'

Instead of rebinding 'C-M-i' to a command that inserts the suggested
completion, we make 'completion-at-point' do the right thing when the
preview is active by temporarily adding a prioritized capf.

Thanks to João Távora for suggesting this approach.

* completion-preview.el (completion-preview-active-mode-map): Remove.
(completion-preview--insert):
(completion-preview-insert-function): New functions.
(completion-preview-active-mode): Adjust.

* completion-preview.el (completion-preview-predicate): Rename

Use a dedicated keymap for when the preview active

* completion-preview.el (completion-preview-hide): New function.
(completion-preview-active-mode-map): New keymap.
(completion-preview-active-mode): New minor mode.
(completion-preview--post-command)
(completion-preview-mode)
(completion-preview-show): Adjust.
(completion-preview-insert-or-complete): Remove.
(completion-preview-insert): New command.

Handle non-zero completion base offset

Compatibility definition for 'minibuffer--sort-by-length-alpha'

* (completion-preview-hook): Fix typo in docstring

Expand Elisp commentary

Add Elisp package headers

Add COPYING file

Add an autoload cookie for 'completion-preview-mode'

Initial commit