Extend IR model and composer API to permit more keywords and flexibility

Composer API

Extend the composer API and IR to improve flexibility and support more C++ language features, in particular

• constexpr, [[attributes]], static on functions / methods
• manually specified function parameter lists
• default and delete
• static class member variables
• separate declarations and definitions for (static) class member variables
• Same inline-ing behavior for constructors as for methods
• Also, it should be possible to expose kernels by adding their declaration to the header file, and to generate inline kernel definitions into the header.

IR

• Extend the IR to support the above concepts