api: add new top-level SignedDuration type #376

Job	Run time
win-gnu	1m 36s
cross (i686-unknown-linux-gnu)	1m 29s
test (stable, ubuntu-latest, stable)	4m 49s
cross (aarch64-unknown-linux-gnu)	1m 37s
msrv	45s
test (beta, ubuntu-latest, beta)	5m 1s
testbench	1m 19s
cross (powerpc-unknown-linux-gnu)	1m 38s
test (nightly, ubuntu-latest, nightly)	3m 15s
cross (powerpc64-unknown-linux-gnu)	1m 21s
test (macos, macos-latest, stable)	2m 54s
wasm32-unknown-emscripten	1m 42s
cross (s390x-unknown-linux-gnu)	1m 54s
wasm32-unknown-uknown	38s
test (win-msvc, windows-latest, stable)	3m 28s
wasm32-wasip1	45s
rustfmt	10s
	34m 21s

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