Make structured Swift representation types Sendable (#2115)

Motivation:

The structured Swift types aren't marked as Sendable. This means that
defining a 'static let' isn't possible, and a static computed property
must be used instead. This is fine, just verbose.

Modifications:

- Make all structured swift representation types 'Sendable'

Result:

Can use 'static let' with structured Swift types

Author: glbrntt