Lots of things to try and get publishing working

[a10y]

Had to change the way we do a few things to get crates.io publishing working for our workspace:

• vortex-build and its use of cargo metadata at compile time was causing lock files to get regenerated, which caused cargo publish to fail. In general, the way pretty much all major projects seem to handle codegen is to do it ahead of time. To that end, vortex-build has been removed completely, supplanted with the xtask setup described below

• Moved all flatbuffer definitions into vortex-flatbuffers. Clients of that crate can select which subset of the definitions they want to compile with using features, current features are:

[features]
dtype = []
scalar = ["dtype"]
array = ["dtype", "scalar"]
file = ["dtype", "scalar", "array"]

• New crate vortex-proto to mirror vortex-flatbuffers but for protobuf definitions. Features on that crate:

[features]
dtype = []
scalar = ["dtype"]
expr = ["dtype", "scalar"]

• Added new xtask binary crate, runnable via cargo xtask -h with commands to execute codegen for both flatbuffers and protobufs. Currently has two subcommands available:

# generate flatbuffers Rust files, written to ./vortex-flatbuffers/src/generated/
cargo xtask generate-fbs

# generate protobuf Rust files, written to ./vortex-proto/src/generated
cargo xtask generate-proto

[a10y]

see https://github.com/matklad/cargo-xtask

this seems like it's on defacto standard in the rust ecosystem for repo automation stuff