Unsafe code is allowed

[lukasvrenner]

TLS requires utmost security. I should #[deny(unsafe)] everywhere except for the C API. #[no_mangle] is unsafe but required. I can create a "safe" version of the public functions, and wrap them in the C functions. I ought to check the generated assembly and make sure the wrapper function is actually optimized away (an extra function call for each function would be silly).