Add UniquePtr::to_shared and SharedPtr::from_unmanaged #5278

Job	Run time
Clippy	0s
Bazel on ${{ (((matrix.os == 'ubuntu') && 'Linux') \|\| ((matrix.os == 'macos') && 'macOS') \|\| ((matrix.os == 'windows') && 'Windows') \|\| '???') }}	0s
Clang Tidy	0s
Outdated	0s
Reindeer	0s
pre_ci / pre_ci	3s
Documentation	26s
Rust nightly	32s
Minimal versions	23s
Rust beta	33s
Rust stable	29s
Rust 1.67.0	1m 7s
Rust 1.70.0	38s
Rust 1.74.0	37s
Cargo on macOS	47s
Cargo on Windows (msvc)	1m 19s
	6m 54s

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