impl `Sync` for `SerialPort` trait #520

Job	Run time
cargo-deny (advisories)	16s
cargo-deny (bans licenses sources)	17s
lint	42s
aarch64-apple-darwin / build	19s
aarch64-apple-darwin-nightly / build	25s
aarch64-apple-ios / build	28s
aarch64-unknown-linux-musl / build	58s
aarch64-unknown-linux-gnu / build	29s
armv7-linux-androideabi / build	25s
i686-pc-windows-gnu / build	2m 10s
arm-linux-androideabi / build	24s
i686-pc-windows-msvc / build	2m 21s
i686-unknown-linux-gnu / build	36s
i686-unknown-linux-musl / build	55s
msrv-aarch64-apple-darwin / build	42s
msrv-arm-linux-androideabi / build	45s
msrv-x86_64-pc-windows-msvc / build	39s
msrv-x86_64-unknown-linux-gnu / build	26s
msrv-x86_64-unknown-freebsd / build	41s
msrv-x86_64-unknown-linux-musl / build	40s
msrv-x86_64-unknown-netbsd / build	36s
x86_64-apple-darwin / build	36s
x86_64-pc-windows-gnu / build	1m 49s
x86_64-pc-windows-msvc / build	2m 5s
x86_64-pc-windows-msvc-nightly / build	1m 11s
x86_64-unknown-freebsd / build	30s
x86_64-unknown-linux-gnu / build	1m 3s
x86_64-unknown-linux-gnu-nightly / build	34s
x86_64-unknown-netbsd / build	22s
x86_64-unknown-linux-musl / build	59s
	24m 23s

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