[WIP] Remove Mapper::Base
[WIP] Remove Mapper::Base

  * Move stuff from Mapper::Base to Mapper::Relation
  * Move #load and #dump from Mapper::Relation to Mapper
  * Move some methods from Mapper to Mapper::Relation
  * Change how spec env is cleared before/after runs
  * Remove various obsolete methods (like finalize_relationships)

We no longer need Mapper::Base since with relation graph it doesn't
matter if it's a base relation under the hood or not. Mappers receive
relation nodes and that's the only thing they care about.

Also Mapper shouldn't be an abstract class as it does provide basic
mapping funtionality minus relation, which means a basic mapper knows
how to load and dump objects but it doesn't know how to fetch them from
external sources. Relation mappers are objects that inherit basic
mapping functionality and they also know how to fetch data using
relation nodes from the relation registry.

This is a WIP. I will work on further clean-up and improve test
coverage.

rom-rb/rom-support