analysis/function: improve API

[XVilka]

Your checklist for this pull request

• I've read the guidelines for contributing to this repository
• I made sure to follow the project's coding style
• I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
• I've added tests that prove my fix is effective or that my feature works (if possible)
• I've updated the rizin book with the relevant information (if needed)

Detailed description

A minor change to use the fixed size unsigned integer for some API outputs, where standard int doesn't make any sense. It makes FFI a bit easier.

Test plan

CI is green