Available for every component

[dbuenzli]

This add an ~available:Features.t argument for each component combinator. It may not make sense on each of these but we can restrict that later. My aim is to first get a good As_project.t data structure (e.g. remove the mutable fields) and corresponding public API to construct it. We can still try to restrict it later, if it turns out to be problematic for Makefile generation.

The commits have more details.

[dbuenzli]

To be clear I'm refining the data structure but I'm not interpreting the changes yet (in the sense actual translation to makefile).