Add Backpack interface for typerep-map

Especially useful when we will expose `Map`-based solution. This will also allow us to refactor tests and benchmarks. We don't need to implement all functions for every solution, for non-interesting one in the internal library we can just have `error "Not supported by this implementation!"`.

Issues on refactoring benchmarks and tests will be open only after this issue is done.

Start by examining the existing typerep-map implementations and the current tests and benchmarks to identify the common operations a Backpack interface would need. Define the shared interface for the Map-based solution and determine which internal implementations can report unsupported operations; completion should make later test and benchmark refactoring possible.

Written by the indexing model from the issue text.