Simplify Mapper

[solnic]

ROM::Mapper still exposes old DSL that is far worse than what ROM::Transformer provides (via Dry::Transformer). Furthermore, we still use mapper builders with all the mapping options to build transformers, which is extra complexity that we don't need anymore.

This should be done prior #531 because adding Identity Map will be simpler with a simplified mapping layer.

TODO

• Remove ROM::Mapper and its usage
• Remove ROM::Header and its usage
• Rename ROM::Transformer => ROM::Mapper
• Refactor ROM::MapperCompiler so that it turns relation AST into a dry-transformer function w/o any intermediate objects as we have now

[flash-gordon]

It is a gem.

[dede999]

I'd like to help with this, but I'm not quite sure about a couple of things,

• There are several definitions of ROM::Mapper and two of ROM::Header. Each of then do you want me to remove? All of them?

[solnic]

@dede999 thanks for offering help. I gotta be honest though - this is one of the biggest and most difficult tasks scheduled for 6.0.0. I'm not sure if it's a good idea to tackle it as the first contribution. Obviously, nothing is blocking you from working on this one, I just want to make sure it's a good match for you. I'll be adding more issues and start using labels that will indicate the level of difficulty. This should make it simpler for people to pick up a suitable task.

[dede999]

I really appreciate your words @solnic . Gonna pick other issue in a future situation