Use core::ffi::CStr and alloc::ffi::CString instead of custom CStr

[benma]

First commit (b37dfdf) message says:

And mark rust_base58_encode_check for testing only, as it is not used in the firmware but only in test_keystore_functional.c.

Can't find that in the code, possibly not included?

That msg is outdated, I ended up changing this and forgot to fix the commit msg. Fixed it now.