feat: Support Dafny tests on test models for Rust

[robin-aws]

Issue #, if available:
Resolves #458

Description of changes:

Supports compiling the common Dafny tests in a test model for Rust. This involved having to change some properties of how we lay out the Rust crates:

• Removed the dafny_impl sub-crate and moved implementation_from_dafny.rs into the main src directory - hence replacing ::simple_boolean_dafny with crate::implementation_from_dafny everywhere. I originally had this separated to better divide Dafny-generated code from Smithy-generated code, but it made implementing externs hard/impossible, and Dafny tests make use of "wrapped services" which are essentially testing-only extern shims.
  • This turned out to be unnecessary, because it is reasonable to patch in additional use ::simple_boolean_dafny::*; and
    use simple_boolean::*; imports into the compiled tests. This is equivalent to the pub use dafny_standard_library::implementation_from_dafny::*; import the Makefile is adding, and will be replaced by a Dafny feature named something like --rust-module-name as some other other supported languages already have.
• Added tests/tests_from_dafny/_wrapped.rs wrapped::client::Client with the implementation of the "wrapped service", which is implementing the Dafny-client interface using the Rust idiomatic client.
  • Since this is only used for testing, but implements a Dafny extern that is only defined by Dafny test code, I guarded this with a wrapped-client feature which the Dafny-compiled integration test enables, as per Activating features for tests/benchmarks rust-lang/cargo#2911 (comment)
• Removed async from the client interfaces - we'd originally kept these for better forwards-compatibility, but AFAICT it's impossible to implement the synchronous Dafny-generated trait methods with async methods. This just means that in the future we'll eventually have to provide separate async clients, but that's happened with the AWS SDKs frequently as well.
  • Worked around this by instantiating a current_thread Tokio Runtime as per https://tokio.rs/tokio/topics/bridging instead.
• Raised the visibility of a few things in the main crate to pub so that the dafny_tests integration test crate can see them - open to feedback here on whether it's reasonable or we should find a better solution.
  • Not necessary any more, see edit to the bullet about the wrapped client.

Not ready to merge yet because this relies on a newer Dafny feature branch commit, which I'll tackle in a separate PR first since it will be noisy to update all the patch files.

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[robin-aws]

The error handling code in here is all over the map, please help me use some cleaner idioms :) I can't use ? in general since r#_Wrappers_Compile::Result is not a Rust Result, but perhaps we can use Result.map_err somehow?

[robin-aws]

Did the best I could with this. :)