Unsoundness in the `Sync` implementation of `Service`

[sosthene-nitrokey]

The stores backed by Littlefs are often not Send (because the Littlefs bindings are not either), so the thread-safety of trussed probably depends on the thread-safety of the store, but there is an unconditional implementation: 

trussed/src/service.rs

Line 208 in e107ed3

unsafe impl<P: Platform, D: Dispatch> Send for Service<P, D> {}

This implementation should be conditional of the !Send implementation of Platform.