Add --names to generate static names for crunched files

[er433]

By default the generated module is accessed by strings, so a bad path compiles fine and fails at run time:

match Assets.read "css/app.css" with Some s -> s | None -> assert false

This PR adds a --names flag that generates a module with one value per crunched file, named after its path, so the compiler checks the reference:

$ ocaml-crunch --mode=plain --names -o assets.ml assets/

Assets.Name.contents Assets.Name.css__app_css

Now, in this way, we'll have a compile time error if passing a "bad path".

The generated module exposes an abstract type with name, path, contents and size accessors plus an all list.

It also carries its own module type S listing just the bindings, which lets a wrapper library re-export the names while hiding everything else... this might be useful when the contents should not be readable directly:

module Files : Assets.Name.S with type t = Assets.Name.t = Assets.Name

I think this covers partially the request of #44 , although I discovered the issue afterwards.

Disclaimer: the changes in this PR were partially made with AI-assisted coding using Claude Code.

[dinosaure]

I think you should definitely take a look on mcrunch. As we said, this package is less and less maintained and we would like to replace it by mcrunch (see #44 for more details).