Add `Sendable` conformances to thread-safe types

[fumoboy007]

All or almost all of Dispatch’s types are thread-safe, so they should be marked as Sendable.

[mman]

There is this weird state in Swift 5.10 where on Linux usage of Dispatch with StrictConcurrencyChecking enabled generates these nonsensical errors that do not exist on macOS.

For example:

main.swift:131:31: warning: passing argument of non-sendable type 'DispatchQueue' outside of main actor-isolated context may introduce data races
        try await host.start(queue: queue)
                              ^
Dispatch.DispatchQueue:1:14: note: class 'DispatchQueue' does not conform to the 'Sendable' protocol
public class DispatchQueue : DispatchObject {
             ^
main.swift:2:1: remark: add '@preconcurrency' to suppress 'Sendable'-related warnings from module 'Dispatch'
import Dispatch
^

This needs fixing...