Standalone code generator

[chrrasmussen]

The Erlang code generator include some changes to the Idris 2 compiler that need to be removed or upstreamed in order for the code generator to become standalone.

The biggest changes are:

• Separate compilation
  • The current solution is very hacky, so I think it needs some improving before upstreaming.
  • Upstreaming also means solving separate compilation for the Scheme and JavaScript code generators.
• Prelude.Types.unpack's current implementation require a string type of fixed-size encoding.
• %cg directives include the namespace they are located in.
• Added a new directory type (Package Directories), which lists directories to search for packages (prelude, base etc.)
• Added more CLI options that are allowed to pass through the package options.