R utilities

• i18n.R: simple class to expose JSON-based internationalization messages.
• import.R: function to source R files as namespaced modules.
• mcpool.R: utilities to expose a pool of connections to fork-based workers via future_promise calls.
• memo.R: utilities to add automatic memoisation capabilities to R6 objects.
• network.R: simple wrapper to make network calls.
• solid.R: utilities to work with SOLID principles in R.