Introduce v1 contexts
Implements #2362 (closed)
This MR introduces Context_v1
(for both memory and disk). It achieves this by functorising the existing context implementation over an irmin-pack configuration.
bfe0246c and 032c3050 rename the 2 Context
modules to Context_v0
. These diffs are not strictly needed, they only exist for consistency with the new Context_v1
modules.