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=item BinHex needs a stateful parser

Unlike its cousins I<base64> and I<uuencode>, BinHex format is not
amenable to being parsed line-by-line.  There appears to be no
guarantee that lines contain 4n encoded characters... and even if there
is one, the BinHex compression algorithm interferes: even when you
can I<decode> one line at a time, you can't necessarily
I<decompress> a line at a time.

For example: a decoded line ending with the byte C<\x90> (the escape
or "mark" character) is ambiguous: depending on the next decoded byte,
it could mean a literal C<\x90> (if the next byte is a C<\x00>), or
it could mean n-1 more repetitions of the previous character (if
the next byte is some nonzero C<n>).

For this reason, a BinHex parser has to be somewhat stateful: you
cannot have code like this:

    #### NO! #### NO! #### NO! #### NO! #### NO! ####
    while (<STDIN>) {            # read HEX
        print hexbin($_);          # convert and write BIN