Switch UNRESOLVABLE_CONDITION to an _Util constant
Refactor ::DBIHacks::_extract_fixed_condition_columns (sequel to 8d005ad9)

Instead of just returning an array of column names, switch to a hashref that
can double as an inferred-value bag. As a bonus deduplicates and folds-in
another codepath from ::ResultSet::_merge_with_rscond

In the process fixup _collapse_cond to be even more robust in some arrayref
corner-cases. Now it is guaranteed to return a hashref at all times
Move a CDBI escape to the CDBI class hierarchy
Another heavy refactor of the rel resolver prototype (sequel to 03f6d1f7)

Change the return value of _resolve_relationship_condition to a 3-value
hash, with a lot of the safety logic consolidated within that method. The
best way to gauge the significance of the changes is to look at the diff
of lib/DBIx/Class/Relationship/Base.pm

Also stop returning the "noncondition-columns" in the _resolve_condition
compa-shim. The information was only used by the code removed from
::Relationship::Base, and is rather new with no evidence of use within
CPAN/DarkPAN. It can be easily added back if necessary.