GROOVY-9381: Add native async/await support (updated proposal)

[paulk-asert]

No description provided.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 78.96440% with 195 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.5986%. Comparing base (165bfd1) to head (af1e9c8).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
.../org/apache/groovy/runtime/async/AsyncSupport.java	67.5497%	84 Missing and 14 partials ⚠️
...apache/groovy/runtime/async/DefaultAsyncScope.java	81.1594%	22 Missing and 4 partials ⚠️
...va/groovy/concurrent/AwaitableAdapterRegistry.java	72.5806%	11 Missing and 6 partials ⚠️
...ache/groovy/runtime/async/DefaultAsyncChannel.java	88.1890%	7 Missing and 8 partials ⚠️
...odehaus/groovy/transform/AsyncTransformHelper.java	78.7234%	7 Missing and 3 partials ⚠️
...va/org/apache/groovy/parser/antlr4/AstBuilder.java	83.0189%	6 Missing and 3 partials ⚠️
...g/apache/groovy/runtime/async/GeneratorBridge.java	86.2069%	7 Missing and 1 partial ⚠️
...org/apache/groovy/runtime/async/GroovyPromise.java	82.8571%	5 Missing and 1 partial ⚠️
src/main/java/groovy/concurrent/AwaitResult.java	93.3333%	0 Missing and 2 partials ⚠️
src/main/java/groovy/concurrent/Awaitable.java	92.8571%	2 Missing ⚠️
... and 1 more

Additional details and impacted files

@@                Coverage Diff                 @@
##               master      #2437        +/-   ##
==================================================
+ Coverage     66.5002%   66.5986%   +0.0984%     
- Complexity      30418      30731       +313     
==================================================
  Files            1420       1435        +15     
  Lines          118577     119504       +927     
  Branches        21044      21159       +115     
==================================================
+ Hits            78854      79588       +734     
- Misses          33204      33355       +151     
- Partials         6519       6561        +42     

Files with missing lines	Coverage Δ
src/main/java/groovy/concurrent/AsyncChannel.java	100.0000% <100.0000%> (ø)
src/main/java/groovy/concurrent/AsyncScope.java	100.0000% <100.0000%> (ø)
...java/groovy/concurrent/ChannelClosedException.java	100.0000% <100.0000%> (ø)
...apache/groovy/reactor/ReactorAwaitableAdapter.java	100.0000% <100.0000%> (ø)
...g/apache/groovy/rxjava/RxJavaAwaitableAdapter.java	100.0000% <100.0000%> (ø)
src/main/java/groovy/concurrent/AwaitResult.java	93.3333% <93.3333%> (ø)
src/main/java/groovy/concurrent/Awaitable.java	92.8571% <92.8571%> (ø)
.../main/java/groovy/concurrent/AwaitableAdapter.java	0.0000% <0.0000%> (ø)
...org/apache/groovy/runtime/async/GroovyPromise.java	82.8571% <82.8571%> (ø)
...g/apache/groovy/runtime/async/GeneratorBridge.java	86.2069% <86.2069%> (ø)
... and 6 more

... and 2 files with indirect coverage changes

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