Parrot's embedding docs

Yesterday I committed a new version of Parrot's embedding docs, now available at http://www.parrotcode.org/docs/embed.html. Most of the document is based on my work with mod_parrot, and reflects the current state of the embedding API rather than any formal spec. Much more to come as I start to verify code coverage, add missing docs, fix missing prototypes, and tweak whatever else creeps up.