Closes #5627

Makes tr operate on bytes only. GNU's behaviour is documented as:

The interpretation of string1 and string2 depends on locale. GNU tr fully supports only safe single-byte locales, where each possible input byte represents a single character. Unfortunately, this means GNU tr will not handle commands like ‘tr ö Ł’ the way you might expect, since (assuming a UTF-8 encoding) this is equivalent to ‘tr '\303\266' '\305\201'’ and GNU tr will simply transliterate all ‘\303’ bytes to ‘\305’ bytes, etc. POSIX does not clearly specify the behavior of tr in locales where characters are represented by byte sequences instead of by individual bytes, or where data might contain invalid bytes that are encoding errors.

We don't use locale yet, but only operate on bytes with this change. This means that unicode characters are no longer supported.