fix: use c_char over i8

[rkrishn7]

c_char is either i8 or u8 depending on the target platform (see https://doc.rust-lang.org/std/os/raw/type.c_char.html)

This fixes compilation problems on architectures like aarch64, for example.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.9%. Comparing base (babb5e7) to head (8b4d62b).

Additional details and impacted files

Files	Coverage Δ
src/rules.rs	96.3% <100.0%> (ø)