[WIP] new ROM relations & mapper extraction

Opening early to discuss.

@snusnu I finally analysed inmemory adapter - there's no way it could work now :) First of all it misses a gateway wrapper that would operate on some in-memory data structure storing materialized relations there. Otherwise you can't make it work since relations are immutable so every time you do, for example, an insert you will get a new one (as you know). Please tell me what was your idea behind in-memory adapter but I think you pre-maturely expected it to just work via base relations (which is impossible)

@dkubb would love to get your input on this (in memory adapter powered by axiom relations)