Rename Mapping => Mapper::Builder
Rename Schema => Schema::Builder
Allow multiple calls to Environment#mapping (inline with schema)
Introduce Environment::Builder and change mapper DSL

This separates environment from its builder DSL and changes mapping DSL
to not use method_missing. Additionally mapping DSL no longer registers
ROM relations and simply builds the mappers. Later on environment builder
uses schema relations and mappers to build final ROM relations.

NOTICE: mapping DSL is still coupled to axiom relation interface and
it's gonna stay like that as long as mappers are responsible for key
attribute introspection (Mapper#{identity, identity_from_tuple}) which
will eventually be extracted into something separate. Just need to find
a good name for it ;)