Greatly improve diagnostic messages of _resolve_relationship_condition
Add internal assertion guard for some indirect calls (for now only create/new)

Modeled on the idea of a9da9b6a, this one blows up when a public proxy
is called insted of the equivalent public chan of methods

This particular set of changes to create() and new() is solely an
optimisation (unlike the next commit)
Replace B::perlstring with our own implmentation thereof

Haarg++ for pointing this out
Standardize the Moo import block, move quote_sub/qsub into ::_Util

This way we will have less boilerplate in subsequent commits where we
will use quote_sub standalone
Reformat add_relationship_accessor for future edit

No functional changes, read diff under -w
Replace many proxy closure-based methods with static qsubs

Not only does this buy a quantum of performance, but it greatly enhances
readability of these methods on deparse

Deliberately not converting the ManyToMany helper - this code needs a
thorough regut :(