Update Orc documentation for language changes (5C)

[arthurp]

The OrcO work changed the core language in several ways that are not related to objects and the documentation needs to be updated to match. Specifically:

• [ ] All discussions of prune need to be removed and replaced with discussions of graft.
• [ ] All discussion of elaboration of site/def calls needs to be updated to the non-killing semantics of "unfolding".
• [ ] Add trim combinator to all combinator discussions and introductions. Add a section covering it as a sibling of other combinators.
• [ ] Check all discussions of "val" binding to make sure it doesn't assume prune (it often did not).
• [ ] All uses of the Let site for kill semantics (a common pattern in examples) should be replaced with trim.
• [ ] Update tutorial to use new language and introduce trim and graft separately.

This issue does not cover the addition of Orc sites or block syntax. See #156 and #158.

[arthurp]

The initial TODO for this work is:

• [ ] Find all references to these elements in the documentation and make a list to make sure we don't miss anything and to get a sense for how the concepts of prune were covered before.
• [ ] Outline how we will cover graft and trim informed by the old prune documentation.

[arthurp]

This may be able to make use of some prose or ideas from the OrcO paper.