Add optional formatting for OP_DSC_FIELD

For some opcodes, the output is not "stable", and depends on the exact
input values; this makes it harder to check consistency against
Haskell code.

To compensate for this, we add a way to override the formatting of the
OP_DSC_FIELD; by default, this is always "%s", but if the
OP_DSC_FORMATTER is defined (must be a callable), it is used to format
the actual value.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>