Introduce Module concept and AST entity

[frame-lang]

Currently, the frame_c parser is organized around parsing a single system. With the introduction of functions and the goal to create fully running programs, Frame needs to introduce some form of compilation unit and a way to execute it.

To this end, the basic grouping of language entities will be a "module". The default container for a module will be a file, though, like Rust, additional syntax will be explored to delineate modules not defined by the contents of a file.