Expose `automerge::AutoCommit::with_actor()` through `AMcreate()`.
Add notes to clarify the purpose of `AMfreeStack()`, `AMpop()`,
`AMpush()`, `AMpushCallback()`, and `AMresultStack`.
source files.
Normalize the header include statement within the documentation.
Limit `AMpush()` usage within the quickstart example to variable
assignment.
quickstart example up to the library as `AMresultStack` so that it can
appear in the README.md and be used to simplify the unit tests.
Promoted `free_results()` to `AMfreeStack()` and `push()` to `AMpush()`.
Added `AMpop()` because no stack should be without one.
