Convert::ascii85 implements the ascii85 (also known as base85) algorithm for encoding binary data as text. this is done by interpreting each group of four bytes as a 32-bit integer, which is then converted to a five-digit base-85 representation using the digits from ascii 33 (!) to 117 (u).

this is similar to mime::base64 but more space efficient: the overhead is only 1/4 of the original data (as opposed to 1/3 for base64).