Second iteration of weights v1.5

[svyatonik]

What has been done here:

• replaced Weight::from_ref_time(0) with Weight::zero() where I'm sure we don't need non-zero value of the second component;
• replaced min_by + ref_time() with Weight::min where appropriate;
• replaced ref_time() == 0 with Weight::is_zer() call where appropriate;
• replaced w1.ref_time() <comparison-op> w2.ref_time with all_lte/all_gte/... calls where appropriate;
• added WeightExtraOps trait for the min_components_checked_div fn that is missing from the Weight struct. We need it to compute number of messages that are fit into the delivery transaction.

Next iteration will be after v2 weights will become available - we'll need to deal with refund, weight formulae, benchmarks, ...