Add `Thread::LinkedList#each` to safely iterate lists #13094

Job	Run time
check_format	1m 35s
x86_64-gnu-test (1.7.3)	31m 49s
x86_64-gnu-test-preview_mt	5m 34s
x86_64-gnu-test (1.8.2)	35m 32s
x86_64-musl-test	35m 32s
x86_64-gnu-test (1.9.2)	31m 56s
x86_64-gnu-test (1.10.1)	34m 1s
x86_64-gnu-test (1.11.2)	35m 4s
x86_64-gnu-test (1.12.2)	33m 12s
x86_64-gnu-test (1.13.3)	30m 53s
x86_64-gnu-test (1.14.0)	31m 46s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	31m 21s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	33m 30s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	32m 32s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	33m 9s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	35m 19s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	33m 42s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	31m 30s
	8h 57m 57s

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