panic instead of raise error on bad spawn

[Guest0x0]

Spawning on calling add_defer on an already terminated task group is an error. Previously this is handled via raising @async.AlreadyTerminated, but this is not the most appropriate solution. This kind of bad spawn are always programmer bug, with very clear pattern. So the most appropriate solution is to panic on bad spawn. This also allows removing the raise mark from the spawn functions, allowing them to be called in contexts that disallow error.

The AlreadyTerminated error become useless now, and is deprecated.

[coveralls]

Pull Request Test Coverage Report for Build 702

Details

• 2 of 4 (50.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.05%) to 78.137%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/task_group.mbt	2	4	50.0%

Totals
Change from base Build 701:	-0.05%
Covered Lines:	1183
Relevant Lines:	1514

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💛 - Coveralls