`Thread` owns its current fiber (instead of `Crystal::Scheduler`) #11278

Job	Run time
check_format	1m 56s
deploy_api_docs	0s
x86_64-gnu-test (1.7.3)	34m 31s
x86_64-gnu-test (1.8.2)	35m 36s
x86_64-gnu-test (1.9.2)	33m 42s
x86_64-gnu-test (1.10.1)	33m 51s
x86_64-gnu-test (1.11.2)	34m 23s
x86_64-gnu-test-preview_mt	5m 53s
x86_64-gnu-test (1.12.1)	34m 10s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	30m 52s
x86_64-musl-test	38m 8s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	35m 19s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	33m 30s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	34m 52s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	33m 7s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	33m 25s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	36m 8s
	8h 9m 23s

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