Partially de-duplicate signatures contained in irmin.mli

[craigfe]

This is a first attempt at inlining the duplicated signatures in Irmin core.
Mostly consists of pushing documentation from irmin.mli to s.ml or
<filename>.mli, and making the alias explicit in irmin.mli:

module type APPEND_ONLY_STORE = sig ... end

(* becomes *)
module type APPEND_ONLY_STORE = S.APPEND_ONLY_STORE

There are three API changes here:

• exposing Type.pp_ty, which was previously unused;

• removing Info.with_message, also unused;

• exposing Merge.with_conlict, which was used internally but not exposed in
  the public signature. I feel as though we might as well provide this.

[craigfe]

I processed the documentation with OCamlformat's parse-docstrings while I was moving it, so a side-benefit of this is that we get much smaller diffs if we ever adopt that option or if it becomes conventional. /cc. @Julow

[pascutto]

I processed the documentation with OCamlformat's parse-docstrings while I was moving it, so a side-benefit of this is that we get much smaller diffs if we ever adopt that option or if it becomes conventional. /cc. @Julow

Yes, let's use this!

[samoht]

LGTM (I've just noticed a weird comment attachement)

And yes, let's use auto-formating in the comments too.

[craigfe]

I've added changelog entries for the newly-exposed functions Type.pp_ty and Merge.with_conflict.