add Bound::as_unbound #9068

Job	Run time
fmt	19s
check-feature-powerset	0s
python${{ matrix.python-version }}-${{ matrix.platform.python-architecture }} ${{ matrix.platform.os }} rust-${{ matrix.rust }}	0s
careful	0s
check-msrv	7m 57s
coverage-windows	6s
docsrs	0s
coverage-macos	7s
emscripten	0s
coverage-ubuntu	1m 18s
semver-checks	2m 35s
clippy/x86_64-apple-darwin/stable	9m 44s
test-cross-compilation ${{ matrix.os }} -> ${{ matrix.target }}	0s
clippy/x86_64-unknown-linux-gnu/stable	9m 15s
test-cross-compilation-windows	0s
test-debug	1m 12s
clippy/powerpc64le-unknown-linux-gnu/stable	8m 10s
clippy/s390x-unknown-linux-gnu/stable	8m 9s
test-version-limits	0s
clippy/wasm32-wasi/stable	7m 48s
valgrind	0s
clippy/x86_64-pc-windows-msvc/stable	9m 25s
clippy/i686-pc-windows-msvc/stable	9m 32s
clippy/x86_64-unknown-linux-gnu/beta	8m 14s
python3.12-x64 ubuntu-latest rust-stable / build	53s
python3.12-x86 windows-latest rust-stable / build	1m 2s
python3.12-x64 windows-latest rust-stable / build	1m 1s
python3.12-x64 macos-latest rust-stable / build	59s
python3.12-x64 ubuntu-latest rust-nightly / build	1m 2s
conclusion	1s
	1h 28m 49s

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