Implement ctx.Identity for C#

[RReverser]

Description of Changes

Fixes #2089.

Since C# doesn't have fixed-size arrays available to safe code, I decided it's simpler to accept a pointer to Identity itself. It's stored as little-endian anyway, so the only change that was necessary is to mark the struct as FFI-compatible. The actual FFI declaration for identity looks a bit different from others for .NET reasons - see comment on the declaration for more details, but the code itself is still pretty simply.

Marking the struct as FFI-compatible also allowed to remove a bunch of helpers, casts and intermediate allocations, since we can now read/write Identity and Address directly instead of reaching into their internal representation.

For testing, I moved the Rust-specific integration test from smoketests to our "standalone integration test" which runs for both Rust and C#, and implemented C# counterpart to it.

API and ABI breaking changes

If this is an API or ABI breaking change, please apply the
corresponding GitHub label.

Expected complexity level and risk

1

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where 1 is a trivial change, and 5 is a deep-reaching and complex change.

This complexity rating applies not only to the complexity apparent in the diff,
but also to its interactions with existing and future code.

If you answered more than a 2, explain what is complex about the PR,
and what other components it interacts with in potentially concerning ways.

Testing

Describe any testing you've done, and any testing you'd like your reviewers to do,
so that you're confident that all the changes work as expected!

• Ran relevant standalone integration tests and fixed the issues.

[gefjon]

Looks great! Thanks for calling out that you moved the test in your PR description; that saved me a lot of confusion.