Optimize nth and nth_back for BoundListIterator #12263

Job	Run time
resolve	3s
fmt	11s
clippy/${{ matrix.platform.rust-target }}/${{ matrix.rust }}	0s
semver-checks	0s
python${{ matrix.python-version }}-${{ matrix.platform.python-architecture }} ${{ matrix.platform.os }} rust-${{ matrix.rust }}	0s
check-msrv	0s
python${{ matrix.python-version }}-${{ matrix.platform.python-architecture }} ${{ matrix.platform.os }} rust-${{ matrix.rust }}	0s
careful	0s
coverage ${{ matrix.os }}	0s
docsrs	0s
valgrind	0s
emscripten	0s
test-debug	0s
Free threaded tests - ${{ matrix.os }}	0s
test-version-limits	0s
check-feature-powerset ${{ matrix.rust }}	0s
test-cross-compilation-windows	0s
test-cross-compilation ${{ matrix.os }} -> ${{ matrix.target }}	0s
conclusion	1s
	15s

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