perf: short circuit in POSIX tz when there is no DST #388

Job	Run time
msrv	44s
win-gnu	1m 33s
cross (i686-unknown-linux-gnu)	1m 4s
test (stable, ubuntu-latest, stable)	4m 42s
testbench	53s
wasm32-unknown-emscripten	1m 27s
cross (aarch64-unknown-linux-gnu)	1m 22s
test (beta, ubuntu-latest, beta)	4m 50s
cross (powerpc-unknown-linux-gnu)	1m 18s
test (nightly, ubuntu-latest, nightly)	3m 12s
wasm32-unknown-uknown	27s
cross (powerpc64-unknown-linux-gnu)	1m 18s
test (macos, macos-latest, stable)	3m 12s
cross (s390x-unknown-linux-gnu)	1m 46s
test (win-msvc, windows-latest, stable)	3m 40s
rustfmt	5s
wasm32-wasip1	45s
	32m 18s

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