Making cosmo optional and questioning the "standalone" version

[Omikhleia]

This module includes a reader, several writers, a command-line converter, and a standalone converter (which vendors C code and some libraries, including cosmo).

This packaging does not seem ideal.
Notably the cosmo dependency is only used in the latter case, if I am not mistaken -- and for sure, not in the reader.
All components however depend on the same util.lua that always require cosmo.

In use cases where one only needs the reader (implementing their own writers/renderers) or doesn't need the templating engine anyway, this forces having cosmo installed.
There should be a way to make it more modular.