Add Backpack interface for typerep-map

[chshersh]

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.

[vrom911]

Do I understand this correctly that this change makes typerep-map library unusable with stack?

[chshersh]

@vrom911 Unfortunately, yes, untill stack adds support for Backpack. Transition to Backpack will break typerep-map into multiple packages:

• typerep-map-sig: Backpack signatures
• typerep-map-containers: containers-based solution
• typerep-map: array-based solution (same name for backwards compatibility reasons)

It's possible to implement benchmarks and tests in separate packages to have typerep-map-containers and typerep-map packages build with stack but don't want to do that.