Customizability Extensions: CFunction Signatures and Code Literals

This MR extends the expressivity of CFunction by allowing users to specify its type signature, and introduces literal expressions.

• CFunction Signature: Instances of CFunction now have to be created with a fixed type signature, which is used by the typifier.
• Literal expressions: Introduce PsLiteral and PsLiteralExpr to allow injection of custom code literals into the AST. Literals must have a fixed type, and are considered constant within the scope of a kernel.

This MR furthermore fixes CFunction printing, and a bug that affected typification and constant elimination in the presence of array initializers.

• Test cases for literals