Use TemplateHaskell to build the opID function

This replaces the hand-coded opID with one automatically generated
from the constructor names, similar to the way Python does it, except
it's done at compilation time as opposed to runtime.

Again, the code line delta does not favour this patch, but this
eliminates error-prone, manual code with auto-generated one; in case
we add more opcode support, this will help a lot.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>