* Drop 19.09 support
This is a year old. NixOS doesn't even support the last stable release,
let alone the one before that.
Also cuts our CI size by a third, which is always nice.
* Remove 19.09 compatibility pieces
It causes a lot of issues. To make this work:
* `shellFor` is updated to use `getAllComponents` instead of `.all`.
* `getAllComponents` is updated to work on the package rather than
the package config.
* Tests updated to not use `.library` or `.exes.X` where appropriate.
* Documentation updated.
* Out of date examples removed.
As a bonus `shellFor` now takes a `components` argument that might be
useful for limiting the dependencies of the shell to just the ones
needed for the components you intend to work on.
Hamish Mackenzie
Arnaud Bailly
Michael Peyton Jones
John A. Lotoski
Arian van Putten
Domen Kožar