Resolve $rsrc instance duality on metadata traversal

Resolve $rsrc instance duality on metadata traversal

Make result_source a straight wrapper around result_source_instance. This
should fix all the fallout introduced in 0.082800 (4006691d), which sadly
went undetected all the way until ~7 months after its release. Ultimately
this is my fault, as I had an early warning, and even later made a conjecture
which spot exactly may blow up in my face (read towards end of 350e8d57)

Exploit the fact that result_source_instance until very recently was a toss-up
between a CAG 'inherited' and a Class::Data::Inheritable (removed in 5e0eea35)
with CAG not really involved in result-instance level calls, and the latter
making it downright impractical due to the closure-based approach. Combined
with the fact that result_source was a 'simple'-type accessor pointing at
the '_result_source' hash-slot allows us (at least seemingly) to switch to a
setup where result_source is nothing but a wrapper around a CAG inherited
accessor result_source_instance which point to the named slot '_result_source'

The changeset is deceptively small, and is kept this way for easier auditing.
See next commit for the armada of additional testing to verify the entire
stack is in fact still solid.