factor out UnboundLoad and rearchitect strategy_options.py

factor out UnboundLoad and rearchitect strategy_options.py

The architecture of Load is mostly rewritten here.

The change includes removal of the "pluggable" aspect
of the loader options, which would patch new methods onto
Load.  This has been replaced by normal methods that
respond normally to typing annotations.  As part of this
change, the bake_loaders() and unbake_loaders() options,
which have no effect since 1.4 and were unlikely to be
in any common use, have been removed.

Additionally, to support annotations for methods that
make use of @decorator, @generative etc., modified
format_argspec_plus to no longer return "args", instead
returns "grouped_args" which is always grouped and
allows return annotations to format correctly.

Fixes: #6986
Change-Id: I6117c642345cdde65a64389bba6057ddd5374427