irmin-pack: compile-time and run-time configurations

[adatario]

Currently when instantiating an irmin-pack.unix store there are two configuration objects that need to be passed in:

1. When instantiating a Maker module using the Irmin_pack.Conf.S module.:

module Maker (Config : Irmin_pack.Conf.S) : Store_intf.Maker_persistent
module KV (Config : Irmin_pack.Conf.S) : Store_intf.KV

2. When v'ing a Repo with a Irmin.Conf.t object:

let* repo = Store.Repo.v 
  (Irmin_pack.config  ~fresh ~index_log_size ~merge_throttle ~indexing_strategy
      ~dict_auto_flush_threshold ~suffix_auto_flush_threshold root)

I'll call the configurations passed to the functor the "compile-time" configurations and the configuration passed to Repo.v as "run-time" configuration.

This is a common pattern in at least one other Irmin backend (irmin-fs).

It seems understandable that there are compile-time configurations and run-time configurations and it makes sense to pack compile-time configurations into an OCaml module as it might contain entire modules (instead of just values).

However, currently compile-time configurations to irmin-pack (and irmin-fs) are just values and the decision what values are compile-time and what values are run-time values seems confusing. For example contents_length_header : length_header is a compile-time configuration whereas index_log_size : int is a run-time configuration.

Possible courses of actions:

• Improve documentation with explanation of run-time and compile-time configurations and rationale why certain configurations are passed at which stage.
• Rename the compile-time configurations to Irmin_pack.Schema. This would be like the Irmin.Schema but with backend specific compile-time configuration.