Provide and use ec_group_get_field_type()

Provide and use ec_group_get_field_type()

All internal uses of EC_METHOD_get_field_type() and EC_GROUP_method_of()
are chained together. Implement this as a single API call that takes a
group and use it throughout. Gets rid of another eyesore in this part of
the tree. Not that there will be a shortage of eyesores anytime soon...

ok jsing