add new build format cep

[wolfv]

This is a mashup of the boa / rattler-build and conda-build documentation for the new recipe spec.

I am not sure in how far we want to spec out things here since there are two speccable things:

• allowed inputs
• expected behavior

This is already a bit of both. What's missing is for example a section on run_exports, including the ignore_run_exports_from and ignore_run_exports. However, when specifying these inputs, we probably also want to specify the behavior a little bit?

I also need to expand more on the expected behavior of outputs and how "merging" works with the top-level recipe.

Todo:

• [ ] document all build keys
• [ ] spec out variant config files and variant evaluation
• [ ] document all available selector variables (unix, win, ...)?
• [ ] figure out if documenting package hash computation is part of this spec or not
• [ ] finish jinja function docs incl. all allowed inputs
• [ ] think more about cmp function

[wolfv]

I've decided to split the proposal up into multiple sub-proposals, the first one is

1. About the new YAML format incl. selectors, allowed Jinja, ... #54
2. The allowed keys / values in the new format (excluding outputs)
3. Specify how variant computation is working
4. The new specification for how outputs should behave
5. The new test magic

If someone thinks that we should split into more parts let me know! :)

[wolfv]

Closing because we've split this CEP into multiple smaller ones :)