Replace Marshal by Granular_marshal in ocaml-index

[voodoos]

Initial PR presentation by @Lucccyo:

The current implementation of ocaml-index uses Marshal to store on the disk the data.
Searching for occurrences on massive projects is time-consuming because the search loads all the data structures from the disk to perform the search.

This Pull Request aims to replace Marshal with a granular version to make the ocaml-index more efficient in reading.
It comes with two granular implementations of the data structures set and map, based on the Stdlib implementation.
During a search operation, the program lazily loads only the required part of the ocaml-index.
It works because the heavy nodes of the granular_map and granular_set have link indirections,
introducing serialization boundaries, which allows Marshal to delay the deserialization of their children.

[voodoos]

On my own testing the indexing time is a slight (5%) increase of the indexing time and no increase in the file size. Fetches are now close to instantaneous and Merlin's memory usage vastly reduced since the index are not fully loaded into memory anymore.

Indexing one large library in Dune:

Benchmark 1: Main
  Time (mean ± σ):      1.116 s ±  0.011 s    [User: 0.941 s, System: 0.159 s]
  Range (min … max):    1.100 s …  1.132 s    10 runs

Benchmark 1: With the new granular-marshall:
  Time (mean ± σ):      1.177 s ±  0.011 s    [User: 1.004 s, System: 0.158 s]
  Range (min … max):    1.162 s …  1.195 s    10 runs

Impact on the larger Dune parallel build appear to be much lower: 1.6% increase.

@liam923 I am going to merge these improvements that make sense for most users, but we should also evaluate the impact of this PR on "larger" codebases.