Avoid extra work during _piqi_ext.ml module loading

This also removes bogus warnings.
Add a new test
Further reduce the surface of piqic-ocaml dependencies on piqilib
Update to the latest Piqi self-spec
Add the ability to specify options for generated print_/prerr_ functions

These are the same serialization options that are passed to
*_piqi_ext.gen_/parse_ functions.
Update piq-config example to use frameless piq
Small code improvements
Merge remote-tracking branch 'upstream/master'