The Rust target should support the 'workers' and 'threading' target properties

[cmnrd]

Currently, the Rust target uses its own custom mechanism for selecting the threaded and unthreaded runtime. Also, the Rust target does not use the current threads property to configure the number of worker threads. Once #290 is fixed, we should also align the Rust target to support the newly introduced workers and threading properties.

[cmnrd]

It turns out that the threads property was already supported by the code generator and runtime. However, it was never tested and in fact the threads target property was not enabled for Rust. #993 renames the property to workers. However, we should revisit the change and add tests. Also support for the threading property is still needed.