Git-like API

• Fix the Irmin_mem backend to work when equal keys might be not
  structurally equal (Pervasives.(=) is evil)
• Fix Hash.SHA1.equal to always return true when the underlying
  bigarrays are equals. Before that, this was only the case when
  the whole Cstruct.t where identical: ie. same bigarray but also
  same offset in the Cstruct.t value, which is obviously not
  always the case. Apply the same fix to Hash.SHA1.compare and
  Hash.SHA1.hash.
• Renamed "tag" to "branch" in the API, as "tag" is confusing for Git
  users. BC.tag is now BC.name and BC.branch is now BC.head_ref.
  The various "Tag" modules are now called "Ref" ("Branch" would be
  confusing here since they only store references to commits, not the
  branch contents).
  Note: The remote HTTP protocol still uses "tag".
• Remove Irmin_http_server.listen. Instead, return the Cohttp
  configuration for the server and let the user perform the listen. The
  resulting API is simpler (removes timeout and uri parameters),
  more flexible, and easier to use from Mirage.
• Remove Irmin.task from API of internal stores (commit, node, etc).
  Tasks are now passed explicitly to operations that need them, so it
  is now explicit which operations create commits. For example, the
  API now makes it clear that lcas doesn't change anything, while
  lca requires a task because it may create commits.
  Apart from simplifying the code, this change also makes it possible to
  create the internal stores once, not once per commit message.
  Note: this does not affect the main BC API, so most users will see no
  difference.
• Remove Irmin.Basic. This was a functor that took a functor for making
  stores and returned a functor for making stores with strings for
  branch names and SHA1 for the hash. It's easier to write the
  application out in full than to explain to people what it does.
  This change also makes it possible for back-ends to provide extra
  operations in a type-safe way. In particular, Irmin_git.Internals
  has moved inside the store type and the runtime check that it is only
  used with the correct store type is now enforced at compile time
  instead.
• Removed AO.config. It was only used by the removed Git.Internals hack.
• Moved AO.create to AO_MAKER.
• Remove dummy functions that are no longer needed with the new API:
  • View.task is gone (it never did anything).
  • View.create is gone (it ignored both its arguments and called View.empty).
  • Ir_node.Graph.Store.create (unused, but previously required by AO).
  • Ir_commit.History.Store.create (same).
• Removed the unused-and-not-exported Ir_bc.Make and Ir_bc.MAKER
  features.
• Combine Ir_bc.STORE_EXT and Ir_s.STORE. Ir_s was the only
  consumer of the Ir_bc.STORE_EXT interface, and all it did was repack
  the values to match its own interface. Now, Ir_bc exports the final
  public API directly, which simplifies the code.
• Moved module types into ir_s.mli and removed ir_s.ml.
  Before, all module types were duplicated in the .ml and .mli files.
• BC stores now contain a Repo module. A Repo.t represents a
  repository as a whole, rather than any particular branch. Operations
  which do not look at the current branch have been moved to this
  module. They are: branches, remove_branch, heads,
  watch_branches, import, export, and task_of_head.
  When updating old code, you can use BC.repo t to get a Repo.t
  from a branch.
  Note that heads previously ensured that the current branch's head was
  included in the returned set (which made a difference for anonymous
  branches). This feature has been removed. In the future, the plan is
  to use OCaml's GC to track which anonymous branches are still being
  used and return all of them.
• The internal stores (commit, node, etc) used to implement a full BC
  store are now created by the back-ends, not by Ir_bc. This allows
  back-ends to use their own APIs for this. In particular, back-ends
  can now share resources (such as a database connection) between
  stores. Internal stores no longer need to deal with config values
  at all.
• Sync.create now takes a Repo.t, not a config, allowing
  Repo.config to be removed and allowing sharing of the back-end's
  internal state with the sync code. For example, the Git back-end
  no longer needs to create a new Git store object for sync.
• Change type head to type commit_id. head was confusing because
  it applied to all commits, not just branch heads. Putting id in the
  name makes it clear that this is just data and (for example) holding
  an ID will not prevent the corresponding commit from being GC'd (once
  we have GC). of_head is now of_commit_id, task_of_head is now
  task_of_commit_id, Internals.commit_of_head is now
  Internals.commit_of_id and BC.Head is now BC.Hash.