investigate / scope adding a module system

[mhuesch]

we could perhaps take inspiration from OCaml's module system (https://dev.realworldocaml.org/first-class-modules.html).

we will likely need to parameterize the module system by the module identfier, so that the language can be used both:

• in a regular filesystem-based context (i.e. where modules are "implemented" using files & directories, where an identifier is a filepath)
• and in Holochain (i.e. where modules are "implemented" using Holochain Entrys, which are identified using EntryHashes)

the latter use represents a kind of "agent-centric" language, where code is not stored in a "data centric" central repository with a global naming scheme. as such it might be kind of novel.