Use smaller Ident type when possible

[Julow]

Add "typed_module", "functor_parameter" and "directmodule" ident and identifier types that are smaller than the corresponding "module" type.

They are used in Lang and Component.

The names are not ideal, especially "direct_module" which has no meaning (it's just a random word)

[jonludlam]

Another possibility we might think about is having the name of the module separate from the parent identifier, so split Module.id into Module.name : ModuleName.t and Module.parent : Identifier.Signature.t - what do you think? Would this be cleaner?

[Julow]

Do we need the name often ? I think keeping identifiers is clearer because most code don't need to think about passing or keeping in sync several fields. eg. they are constructed at one place and deconstructed in specific places too. However in Component.Signature, we sometimes need the name, which is the main motivation for this change.

[jonludlam]

After some discussion elsewhere, this is what we think:

• Identifier.Module.t should be just `Root of Root.t * ModuleName.t | Module of signature * ModuleName.t -- note that this is using ModuleName.t rather than UnitName.t
• There should be a new Identifier.Parameter.t that is just `Parameter of signature * ParameterName.t
• There should be a new Identifier.FunctorResult.t which is `Result of signature
• Identifier.Path.Module.t should be extended to be [ Identifier.Module.t | Identifier.Paramter.t | Identifier.FunctorResult.t ]

This enables functions on every signature element akin to Module.t -> ModuleName.t, ModuleType.t -> ModuleTypeName.t and so on.

The changes should also be reflected in the Components type definitions.

[Julow]

I implemented this.

[Julow]

Removed the unecessary coercions (they were left overs from trying to constraint the types further)

[jonludlam]

The build was failing earlier on scratch.md - you can just nuke that file, it was just for experimentation

[jonludlam]

Great thanks!