Update to [email protected], implement marks (#39)

* Update to [email protected], implement marks

There are some mechanical changes to the API:

* Allow negative `delete` arguments for `Doc.splice` and
  `Doc.spliceText`
* Add `Doc.mark`, `Doc.marks` and `Doc.marksAt`
* Add the `Mark` and `ExpandMark` types
* Add PatchAction.Conflict and PatchAction.Marks
* Add a `marks` field to `PatchAction.SpliceText` and
  `PatchAction.Insert`

One slightly more significant change is the addition of `unsafe impl
Send{}` and `unsafe impl Sync{}` on the `Doc` we export to uniffi. This
is required because currently `automerge::Automerge` is not `Send` or
`Sync` due to the internal use of an `Rc`. The `unsafe` impls are okay
because we guarantee that all accesses of the document on the Swift side
are from a single thread. In future we should be able to remove the `Rc`
from automerge to make it `Send` again and remove these unsafe impls.

* fix: removes extraneous whitespace `rustfmt` complains about.
* fix: sets LOCAL_BUILD environment variable in CI so that
swift package uses the locally built XCFramework. Thought this was
working, but with errors on the swift file, I think I missed it earlier.
* docs: updates documentation to curate Marks into Document and overall Automerge-Swift documentation

---------

Co-authored-by: Joe Heck <[email protected]>

.github/workflows/ci.yaml

@@ -3,6 +3,8 @@ on: [push]
 jobs:
   build-test:
     runs-on: macos-latest
+    env:
+      LOCAL_BUILD: true
     steps:
       - uses: actions/checkout@v3
       - uses: actions-rs/toolchain@v1