Sometimes you need a cheap copy of a document at a given set of heads
just so you can see what has changed. Cloning the document to do this is
quite expensive when you don't need a writable copy. Add automerge.view
to allow a cheap read only copy of a document at a given set of heads
and add an additional heads argument to clone for when you do want a
writable copy.
Continuing our theme of treating all languages equally, move
wrappers/javascript to javascrpit. Automerge libraries for new languages
should be built at this top level if possible.
