seal: avoid hitting the trie when computing weight

[pepyakin]

For computing the weight cost of put_code we fetch the schedule from the storage. However, accessing the storage within the validate_transaction path is discouraged. That's why we should consider switching the put_code_per_byte_cost to be a const.

[athei]

This will be solved for free once the proper weights arrive. The prices for the extrinsics are then read directly from the WeightInfo Trait and not from the schedule.