stoikov: Pass midquote and strategy types to functions

Signed-off-by: Aleksi Pitkäjärvi <aleksi.pitkajarvi@aalto.fi>
stoikov: Pass strategy type to Spread::update; Use enum classes

Signed-off-by: Aleksi Pitkäjärvi <aleksi.pitkajarvi@aalto.fi>
stoikov: Pass midquote type to Midquote constructor

Signed-off-by: Aleksi Pitkäjärvi <aleksi.pitkajarvi@aalto.fi>