Allow zzcat to output immediately

[Sam Hocevar]

If a zzcat sequence doesn't use rewind() or fseek(), we know that any read byte can be immediately sent to the standard output. This would improve zzcat's speed and memory usage:

• design a simple bytecode language
• write another parser that translates the sequence to bytecode
• check for syntax BEFORE running the bytecode!
• if rewinding functions don't appear in the bytecode, output characters immediately