Function complement_rna 

pub fn complement_rna(base: u8) -> u8

Complement a single nucleotide as RNA: A becomes U, not T.

assert_eq!(complement_rna(b'A'), b'U');
assert_eq!(complement_rna(b'U'), b'A');
assert_eq!(complement_rna(b'T'), b'A');
assert_eq!(complement_rna(complement_rna(b'u')), b'u');