[Newcompiler] Finding enough bytecodes for closures.

[bryce at kampjes.demon.co.uk]

There's 2 unused bytecodes and some holes in the double extended
do anything bytecode. That's enough for most designs. I'm guessing
that a good implementation will involve 4 bytecodes. The holes
in the do anything bytecodes is because only 2 of the 8 subtypes
use the 5 bits of argument count from the second byte with the
sub-type.

We'd need to check before borrowing the argument bits that they
are always zero in the image when they're not needed.

Bryce