api: add new top-level SignedDuration type #381

Job	Run time
win-gnu	1m 41s
msrv	49s
test (stable, ubuntu-latest, stable)	4m 47s
cross (i686-unknown-linux-gnu)	1m 1s
testbench	54s
wasm32-unknown-emscripten	1m 26s
test (beta, ubuntu-latest, beta)	4m 55s
cross (aarch64-unknown-linux-gnu)	1m 23s
test (nightly, ubuntu-latest, nightly)	3m 12s
cross (powerpc-unknown-linux-gnu)	1m 21s
test (macos, macos-latest, stable)	3m 23s
wasm32-unknown-uknown	23s
test (win-msvc, windows-latest, stable)	3m 55s
cross (powerpc64-unknown-linux-gnu)	1m 20s
wasm32-wasip1	45s
cross (s390x-unknown-linux-gnu)	1m 38s
rustfmt	6s
	32m 59s

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