Refactor Makefile.toml into smaller files

[sffc]

Makefile.toml is currently a grab-bag of a lot of different build tasks, and it keeps growing. It would be better if we could refactor separate parts of it into separate files that get included into the main file.

I believe that this would also enable us to use Duckscript functions, allowing us to reduce code duplication.

This could involve a new top-level directory named build_scripts or similar.

[dminor]

We should move README.tpl at the same time, in case people confuse it with an actual readme.

[sffc]

@dminor : tools/scripts ?

2021-05-07: GO with tools/scripts.

[sffc]

As part of this, I'd like to fully unify the tests run on CI with the ones in Makefile.toml. We have taken great steps toward unifying them, but we still have the problem where a different subset of tests runs on CI versus what runs in Makefile.toml.

[sffc]

Splitting the Duckscript functions into #785