It was probably a thinko in d00e99e7 (Add more opcodes so we can read
Graphite's Carbon stream) which changed opcodes from string ("X") to
character ('X') constants and marked the first opcode with byte type,
probably with the idea that following opcodes will have the same type.

Unfortunately it is not so as the following program demonstrates:

	package main

	const (
	        opAAA byte = 'a'
	        opBBB      = 'b'
	)

	func main() {
	        op := opBBB
	        if true {
	                op = opAAA	// <-- line 11
	        }
	        println(op)
	}

	--> ./bc.go:11:6: cannot use opAAA (type byte) as type rune in assignment

Similarly if we try comparing opcodes, it also results in compile error:

	func main() {
	        op := opBBB
	        if op == opAAA {	// <-- line 10
	                panic(0)
	        }
	        println(op)
	}

	--> ./bc.go:10:8: invalid operation: op == opAAA (mismatched types rune and byte)

Since in the following patches it will be handy for encoder to e.g. set default
opcode first, and then change it under some conditions to another opcode,
exactly the same compile error(s) will pop up.

So let's fix all opcode constants to have their type as byte to avoid such
unexpected errors.