Fix PdCString::from_str on unix #231

Job	Run time
examples (beta, run-app)	1m 24s
examples (beta, run-app-with-args)	1m 38s
examples (beta, call-managed-function)	1m 23s
examples (beta, passing-parameters)	1m 37s
examples (beta, return-string-from-managed)	1m 33s
test (beta, x86_64-unknown-linux-gnu, 6.0)	5m 0s
test (beta, x86_64-unknown-linux-gnu, 7.0)	4m 17s
test (beta, x86_64-unknown-linux-gnu, 8.0)	4m 24s
test (beta, x86_64-pc-windows-msvc, 6.0)	5m 44s
test (beta, x86_64-pc-windows-msvc, 7.0)	4m 37s
test (beta, x86_64-pc-windows-msvc, 8.0)	5m 38s
test (beta, i686-pc-windows-msvc, 6.0)	5m 14s
test (beta, i686-pc-windows-msvc, 7.0)	5m 55s
test (beta, i686-pc-windows-msvc, 8.0)	6m 3s
test (beta, x86_64-apple-darwin, 6.0)	5m 58s
test (beta, x86_64-apple-darwin, 7.0)	6m 13s
test (beta, x86_64-apple-darwin, 8.0)	5m 49s
clippy (ubuntu-latest)	1m 3s
clippy (windows-latest)	1m 5s
cross (beta, aarch64-unknown-linux-gnu)	3m 5s
cross (beta, armv7-unknown-linux-gnueabihf)	2m 59s
cross (beta, aarch64-pc-windows-msvc)	2m 39s
documentation (ubuntu-latest)	2m 27s
documentation (windows-latest)	4m 33s
fmt (ubuntu-latest)	13s
fmt (windows-latest)	32s
	1h 31m 3s

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