add a way to map contents to a subtree

[samoht]

This will be useful to map OCaml values of a given type to a subtree. Design idea: look at views in ctypes.

[samoht]

Seems related to #6

[samoht]

Done in #43

I've added the concept of "views", which looks like a normal read-write store, but can be used to:

• cache a subtree of store by fetching lazily the entries being read
• write only in-memory, without having to compute and (expansive) key from the value contents

A view can be built in two ways:

• start by an empty view, add contents using the usual Irmin.RW functions
• root a view to a node in a given store. All the reads will be fetched from the store underneath.

The you can "commit" a view anywhere in the store (ie. under any path). At the moment, the view contents will replace the previous contents stored under this path. You can store the same view in different places.