implement Send and Sync for Ftdi

The auto traits `Send` and `Sync` were not implemented because `Ftdi`
contains a (type aliased) raw pointer (`FT_HANDLE`). `Send` and `Sync`
are now implemented explicitly, making it possible to use `Ftdi` in
different threads.

CHANGELOG.md

@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 ### Changed
 - Updated the edition from 2018 to 2021.
+- Implement `Send` and `Sync` for `Ftdi`.
 
 ## [0.32.0] - 2021-11-07
 ### Added

src/lib.rs

@@ -2223,6 +2223,15 @@ impl Drop for Ftdi {
     }
 }
 
+// Safety: Unfortunatly there is no clear statement about the thread safety of
+// the D2XX library in the official programming guide. But there are mentions
+// about thread-safety fixes in the release notes of the driver, which suggests
+// that the library is indeed thread synchronized. It would also be quite
+// strange if the D2XX driver could not be used across threads, but the VCP
+// driver can.
+unsafe impl Send for Ftdi {}
+unsafe impl Sync for Ftdi {}
+
 macro_rules! impl_boilerplate_for {
     ($DEVICE:ident, $TYPE:expr) => {
         impl FtdiCommon for $DEVICE {