* Use AMbyteSpan for byte values
Before this change there was an inconsistency between AMmapPutString
(which took an AMbyteSpan) and AMmapPutBytes (which took a pointer +
length).
Either is fine, but we should do the same in both places. I chose this
path to make it clear that the value passed in was an automerge value,
and to be symmetric with AMvalue.bytes when you do an AMmapGet().
I did not update other APIs (like load) that take a pointer + length, as
that is idiomatic usage for C, and these functions are not operating on
byte values stored in automerge.
Transactions with no ops in them are generally undesirable. They take up
space in the change log but do nothing else. They are not useless
though, it may occasionally be necessary to create an empty change in
order to list all the current heads of the document as dependents of the
empty change.
The current API makes no distinction between empty changes and non-empty
changes. If the user calls `Transaction::commit` a change is created
regardless of whether there are ops to commit. To provide a more useful
API modify `commit` so that if there is a no-op transaction then no
changes are created, but provide explicit methods to create an empty
change via `Transaction::empty_change`, `Automerge::empty_change` and
`Autocommit::empty_change`. Also make these APIs available in Javascript
and C.
After some discussion with PVH I realise that the repo structure in the
last reorg was very rust-centric. In an attempt to put each language on
a level footing move the rust code and project files into ./rust
