Generate async server implementation for Swift

This originally comes from https://github.com/apache/thrift/pull/1997, raised by
David Furey in January 2020, which added support to the compiler for generating
interfaces which allow Swift servers to respond to client requests asynchronously.
Although that PR wasn't merged, we have maintained our fork of the Thrift library
as https://github.com/guardian/french-thrift, and occasionally tweaked the code
to keep up with changes.

This single commit contains the 3 commits from David's original PR, along with:

* 73bca53db849ab0afdde264731813ba85cfc51f0 - Hash (into)
* c87b89cf1a08877ba0764357603b32802a452ce6 - Use stdlib Result instead of TAsyncResult
* 94cccfda5e2713572b560a024df5f8a9d558b7dd - Swift 5.1 support

Co-authored-by: Simone Smith <[email protected]>
Co-authored-by: David Furey <[email protected]>
Co-authored-by: Sophie Lambrakis <[email protected]>
Co-authored-by: Alexander Edge <[email protected]>

Merge pull request #13 from guardian/thrift-0.19

Fork from `apache/[email protected]` for the `mobile-apps` branch

Merge pull request #3 from guardian/swift-deprecations

Swift 5 deprecations