makefile: doc-build: improve compat with BSD sed
Spruce up; improve project metadata, docs

* convert to a Spruce project.  rename project and root package
  accordingly.
* update refs to Spruce libs.
* improve readme.
* add a changelog.
* generate long description from readme and changelog.
Converter.__init__(): fix typo
improve repr

AnnotatedType.convertible_value_description(): improve default value.
improve style
update copyrights