Sketch the new frontend component translator (no inlining) producing HIR2-style IR structure

[greenhat]

This PR introduces ComponentTranslator2 producing HIR2-style IR Component with export interfaces, etc. The HIR2-style Component is roughly sketched in the hir2_sketch module after the HIR2, and will be discarded in favor of the real HIR2 when it's available.

TODO:

• CanonLower synthetic functions (component imports) should be implemented in the same module where they are called;
• CanonLift synthetic functions (component exports) should be implemented in the Interface;
• sketch inner calls in the synthetic function bodies;
• address TODOs;
• document code;