This transforms the 'Command' tuple into a record which is
easier to extend.

While at it, this refactoring turns the IDE `:complete` into a hidden
command excluded from completion.

The next obvious step is to add a summary text field for constructing
the `:help` output (as well as allowing to get `:help <CMD>` for single
commands.

This is a preparatory refactoring for D1240 / #10874

Reviewed By: thomie, bgamari

Differential Revision: https://phabricator.haskell.org/D1590