First version of the backmp11 backend.
Contains:
- favor_runtime_speed policy that mainly replaces MPL with Mp11
- favor_compile_time that uses a new mechanism with dispatch tables per state
* Extracted traits into a separate header to include multiple backends at once
* Refactored tests to test multiple variations of backends with the same test code
* Added test cases with back::favor_compile_time for existing tests
* Removed redundant Back11 tests
