Refactor function generation

[xlauko]

At the moment the logic is on three places:

• CodeGenDriver, CodeGen and CodeGenDeclVisitor
• driver and visitor also duplicates some logic -- emit_function_prologue and VisitFunctionDecl
• emit_function_prologue should not really exist

Ideally we want to have separated responsibility:

• CodeGenDriver to parse frontend options
• CodeGen orchestrate scoping
• probably best option would be to extract separate component to handle funciton codegen -- move emit_function_prologue and start_function from CodeGen ?
• CodeGenDeclVisitor orchestrate mlir generation