Rust subworker

[gavento]

Locally tested rust subworker with many tests, some finished APIs, docs.

Must be tested with: cargo test -- --test-threads=1 (parallel tests clash their working dir settings)

Missing/WIP pieces include:

• [x] update on mutability/shared access to outputs and context from the task function
• [x] macros to add task functions with positional attributes
• attributes and content types with file/directory checking
• [x] (mostyl) more helper methods to Context and possibly DataInstance/Output.
• [x] build+testing in CI

[gavento]

@spirali Would you please take a look and test it against the worker? I am on a weak connection for the next few days and that complicates things. Also, feedback is welcome! (along with integration tests ;)

[spirali]

I have put basis for integration testing of rust subworker here: https://github.com/spirali/rain/commit/bc4cea8a44c9d1333a9fad898af51baa8410f773

• The tests expect that "rain-task-test" is built.
• I have merged some of my code for cpp subworker to avoid conflicts in testing.
• Test example is in test_rustsw.py

I will do some more review later.

[spirali]

I suggest that we could standardize some operations that should by implemented by tester of subworker library (e.g. hello world, a kind of concat, return object without change, etc). This allows us to simply reuse one set of basic python tests for all subworkers.

[gavento]

Almost everything not dealing with attributes is finished and somewhat polished.

New and shiny docs and examples, many tests, flexible registration with register_task!, simplified writing to Output (no more built-in locks). There are still TODOs. Feedback welcome.

[gavento]

@spirali That sounds reasonable, do you want to open an issue/PR with a summary of the requirements, or make a writeup doc?