refactor process API, integrate with native task system & cancellation

[Guest0x0]

This PR refactors the API of @process. Previously, the API is non-structural. The spawned process is orphan, and may leak if not properly waited. The previous API cannot integrate with structured concurrency as well.

This PR replaces @process.spawn with @process.run, Compared to @process.spawn, @process.run will wait until the spawned process completes. And if current task is cancelled, the spawned process will be terminated via SIGTERM, making process spawning structural.

Previous behavior can still be recovered partially:

• to run the process in background, just use spawn or spawn_bg
• to prevent the process from being killed, pass orphan=true to @process.run

[coveralls]

Pull Request Test Coverage Report for Build 112

Details

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.7%) to 81.517%

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Totals
Change from base Build 111:	0.7%
Covered Lines:	516
Relevant Lines:	633

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💛 - Coveralls