Adds enough to build the AST nodes for the module function declarations in a .em file.
Adds:
- Function: base class for module function declarations and user functions
- FunctionParameterDeclaration: declaration for a function parameter, as well as its default value if any
- FunctionParameterList: just a holder for function parameter declarations
- ModuleFunctionDeclaration: for function declarations in .em files
- FunctionResolver: hooks up function calls to module functions or user functions
- ModuleProcessor: a visitor for processing const declarations and module function declarations (const declarations are yet to come from the main branch)
