Prepare v0.12.1 release #340

Job	Run time
Check	23s
Check doc links	18s
Miri ubuntu-latest	1m 33s
Clippy	29s
Miri windows-latest	2m 28s
Rustfmt	13s
Miri macos-latest	1m 34s
address sanitizer	1m 3s
memory sanitizer	1m 12s
Test stable on ubuntu-latest	2m 7s
thread sanitizer	1m 2s
Test nightly on ubuntu-latest	1m 37s
leak sanitizer	57s
Test stable on windows-latest	3m 26s
Test nightly on windows-latest	3m 12s
Test stable on macos-latest	1m 45s
Test nightly on macos-latest	1m 35s
	24m 54s

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