refactor(ffi): simplify ffi API

[notmandatory]

Description

Simplified FFI API to use strings for arguments and return values. Also removed parameters like SATSCARD new slot when the lib can figure out on it's own which one to use.

Notes to the reviewers

Thanks to @reez for helping me make this simpler.

Changelog notice

• refactor FFI to use simpler API that uses strings for arguments and return values.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

[reez]

Built this PR bindings successfully locally. Imported those local bindings into the Xcode project successfully.

Haven't used any of the big api changes (but wasn't using them previously yet in the app yet either) yet, but looking at the code changes they look right from what we discussed yesterday.