The previous approach of using the key and insert columns of existing
ops was leading to quite confusing code. There's no real cost to
introducing new columns so I've switched the code to do that instead.
Introduce an `expand` and a `mark_name` column. `expand` is a boolean
column and `mark_name` is a RLE encoded string column. Neither of these
columns are encoded if they are empty.
Also move the `MarkData::name` property to use strings interned in
`OpSetMetadata::props` rather than representing the string directly on
the basis that we probably will have a lot of repeated mark names and
we do a bunch of equality checks on them while searching so this will
probably speed things up a bit.
Introduced a new `MaybeBooleanEncoder` (and associated `MaybeBooleanDecoder` and
`MaybeBooleanRange`) types to represent a boolean column which is
entirely skipped if all it contains are `false` values. This allows us
to omit encoding the `expand` column for groups of ops which only ever
set it to `false` which in turn makes us backwards compatible when not
using marks.
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
