api: add new top-level SignedDuration type #384

Job	Run time
cross (i686-unknown-linux-gnu)	1m 4s
cross (aarch64-unknown-linux-gnu)	1m 23s
cross (powerpc-unknown-linux-gnu)	1m 39s
wasm32-unknown-emscripten	1m 30s
cross (powerpc64-unknown-linux-gnu)	1m 20s
msrv	46s
cross (s390x-unknown-linux-gnu)	1m 46s
test (stable, ubuntu-latest, stable)	4m 39s
wasm32-unknown-uknown	24s
wasm32-wasip1	49s
test (beta, ubuntu-latest, beta)	5m 1s
win-gnu	1m 30s
test (nightly, ubuntu-latest, nightly)	3m 21s
rustfmt	14s
test (macos, macos-latest, stable)	2m 56s
testbench	52s
test (win-msvc, windows-latest, stable)	3m 58s
	33m 12s

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