api: add new top-level SignedDuration type #395

Job	Run time
rustfmt	5s
cross (i686-unknown-linux-gnu)	1m 0s
msrv	51s
test (stable, ubuntu-latest, stable)	4m 51s
cross (aarch64-unknown-linux-gnu)	1m 26s
test (beta, ubuntu-latest, beta)	4m 58s
testbench	56s
cross (powerpc-unknown-linux-gnu)	1m 22s
wasm32-unknown-emscripten	1m 37s
test (nightly, ubuntu-latest, nightly)	3m 11s
cross (powerpc64-unknown-linux-gnu)	1m 21s
test (macos, macos-latest, stable)	3m 16s
cross (s390x-unknown-linux-gnu)	1m 56s
wasm32-unknown-uknown	33s
wasm32-wasip1	46s
test (win-msvc, windows-latest, stable)	3m 53s
win-gnu	1m 33s
	33m 35s

Provide feedback