Implement opcode summary support

This implements in the Haskell codebase the opcode summary. As opposed
to Python, we always use custom code for formatting, since we don't
want to use dynamic attribute lookup.

To test this properly, we need to change MetaOpCode to record-syntax,
so that we get accessor functions.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>