Optimizie OpCode loading

This patch converts the opcode loading to a pre-built map (at import
time) instead of iteration over the globals dict at each call.

Microbenchmarks show that this should be around three times faster, and
burnin still passes.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>