Add a formal notion of group

[Drup]

My design idea is the following:

The combinator group : Group.t -> 'a impl -> 'a impl would take an implementation and bundle it in the given group. It needs to copy the underlying implementation (to handle things like let a = group g x in let b = group g y in ...). We have Group.t = { prefix : string ; name : string }, prefix is for the keys.

This would allow to

• Prefix automatically all the subkeys in an implementation
• Improve documentation by using graphviz's subgraph property.

I'm not completely sure how to implement that just yet. The interaction with sharing in particular is a bit tricky. This is emulated in mirage, which prove that it's very desirable.

[Drup]

This feature is starting to get more desirable for more than just keys.

Currently we have the following constructs:

• A configurable, with some dependencies and keys.
• If
• Application

This doesn't allow encoding various things cleanly, such as

• Wrapping around a device.
• Specifying dependencies to/from a group of devices.

I think the grouping could also solve those issues, by allowing to specify dependencies to a group of device directly.