Combine Cargo workspaces

[andygrove]

• Ballista is now part of the default workspace
• Updated Ballista version from 0.4.2-SNAPSHOT to 0.5.0-SNAPSHOT since the next release is significant as first from the new repo
• I did not change the DataFusion version and we will need to do that at some point (4.1.0 or 5.0.0 next?)

Closes #17

[andygrove]

@alamb @jorgecarleitao Are you ok with this approach of having a single Cargo workspace for all crates? The dependency is still that Ballista depends on DataFusion.

@Dandandan This also removes snmalloc as a default feature so that we don't require CI and all DataFusion devs to compile with snmalloc. My thinking is that it makes sense to build Ballista executors with snmalloc when creating the Docker images. Let me know what you think.

[codecov-commenter]

Codecov Report

Merging #23 (f95bd80) into master (c365a4f) will increase coverage by 5.42%.
The diff coverage is 82.20%.

❗ Current head f95bd80 differs from pull request most recent head f961577. Consider uploading reports for the commit f961577 to get more accurate results

@@            Coverage Diff             @@
##           master      #23      +/-   ##
==========================================
+ Coverage   70.41%   75.83%   +5.42%     
==========================================
  Files         123      135      +12     
  Lines       21261    23152    +1891     
==========================================
+ Hits        14970    17557    +2587     
+ Misses       6291     5595     -696     

Impacted Files	Coverage Δ
ballista/rust/client/src/context.rs	0.00% <0.00%> (ø)
ballista/rust/core/src/datasource.rs	0.00% <ø> (ø)
ballista/rust/core/src/error.rs	0.00% <ø> (ø)
...sta/rust/core/src/serde/logical_plan/from_proto.rs	40.56% <ø> (+40.56%)	⬆️
...llista/rust/core/src/serde/scheduler/from_proto.rs	0.00% <ø> (ø)
ballista/rust/core/src/serde/scheduler/mod.rs	14.28% <ø> (+14.28%)	⬆️
ballista/rust/core/src/serde/scheduler/to_proto.rs	0.00% <ø> (ø)
datafusion/src/optimizer/projection_push_down.rs	98.66% <ø> (ø)
datafusion/src/physical_plan/hash_utils.rs	100.00% <ø> (+2.89%)	⬆️
datafusion/src/optimizer/constant_folding.rs	91.95% <33.33%> (-0.36%)	⬇️
... and 70 more

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[alamb]

In general, I think this approach of a single workspace is fine, as I expect most time people who are working on only datafusion can run cargo test -p datafusion and avoid the build of ballista if they so desire.

[jorgecarleitao]

LGTM

I understood that a cargo workspace is just a way to have a common dependency resolution tree (a common Cargo.lock for all projects), see here, and some other smaller things.

I have not yet fully understood the consequences of this observation to conclude whether a workspace for all projects is useful for us or not; just a neutral information / observation. ^_^