Improve the context API

This MR improves the lib_storage/context.ml API, to get closer to the Irmin API.

Depends on:

• !2510 (merged) (Irmin 2.3 release)
• !2461 (merged) (refactor lib-storage signatures)

This MR introduces a new context API, initially only for the shell but the next MRs will push this API through the protocol.

The main changes are:

• Context.fold is now recursive ;
• a Context.Tree modules which is almost a subset of the corresponding Irmin.Tree API.
• the copy function has been removed as using trees directly is more efficient.
• A pagination-friendly Context.list function has been added.
• A way to expose context subtrees to raw, in-memory subtrees has been added (and the relevant data-encoding). This is useful for the proxy code (see !2466 (merged))