NFC: simplify C# FFI definitions

[RReverser]

Description of Changes

This is non-functional change, just aligning FFI definitions closer to the language conventions.

C# uses int everywhere for positons and lengths in standard APIs.

Our manual conversions from Rust uint to C# int do the same bitcast anyway, so we might as well change the FFI definitions instead for cleaner code and fewer casts.

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

How complicated do you think these changes are? Grade on a scale from 1 to 5,
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 dotnet test, waiting for CI for the rest.