`[inputs]` is now a TOML Table (a key/value store). Each input is
represented in the `[inputs]` table by a key/value pair.
The key is a unique name (which also maps to the recipe file for building
that input thing).
The value is another table describing that input, that we'll represent
with an Input struct.
Inspect with `tomlq . peristaltic-pump.recipe`.
I don't like how i have to name the intermediate subassemblies.
And the the toml syntax for "array of tables" feels pretty clunky:
<https://toml.io/en/v1.0.0#array-of-tables>
