Code length calculation broken w.r.t. EXTENDED_ARG opcodes

[smontanaro]

In the block level representation of the code, Instructions don't require EXTENDED_ARG opcodes. Calculating block and instruction length at that level doesn't take them into account. Inserting them into the instruction stream in the bytes method tosses a wrench into things. There are probably a couple ways to fix this. Just need to figure out which is best and implement.

[smontanaro]

I see a couple ways to deal with this.

1. Take EXT_ARG into account when computing __len__, even though they aren't present at that point. You'd have to recompute them every time __len__() is called.
2. Always assume a full complement of EXT_ARG opcodes, padding oparg with zeroes, then optimize out EXT_ARG 0 instances later (which would require jump offset patch-up).

Option 1 seems the more foolproof option at this point.