io::File: Convert file-name strings from Rust to C
io::File: Convert file-name strings from Rust to C

Rust strings are not terminated by a \0 character. Passing them
directly to a C interface yields an undefined result. This patch
adds string conversion to io::File.

fabricedesre/cc3200-rs,fabricedesre/cc3200-rs,fabricedesre/cc3200-rs,fabricedesre/cc3200-rs,fabricedesre/cc3200-rs