Cmp document functions

Add t::lib::PPI::Test::CMP, containing the cmp_document family of testing functions, plus conversion of one .t file to use them. This is the last of the material from the old cmp_document branch. When it is merged, the old pull request #49  can be closed.

Changes since this material was last reviewed: added Class::Name => content shorthand, ability to skip children of an element via NODESCEND.