seal: Refactor ext_gas_price

[athei]

Subset of: #6355

Overview

This PR changes the signature of ext_gas_price from

fn ext_gas_price() -> BalanceOf<T>

to

fn ext_gas_price(gas: Gas) -> BalanceOf<T>

In addition to that we remove the dependency of seal on pallet_transaction_payment by adding those two elements to the Trait:

/// The currency in which fees are paid and contract balances are held.
type Currency: Currency<Self::AccountId>;

/// Used to supply contracts with the current gas price. This is **not** used to
/// calculate the actual fee and is only for informational purposes.
type WeightPrice: Convert<Weight, BalanceOf<Self>>

Motivation

There is no longer something like gas. We merely keep the term around for historical reasons. Costs are measured in weight. Currently, we provide gas price as the costs for one weight. Calculating the costs for a single weight is subject to rounding errors because the costs for one weight can be below one. In addition to that a fixed point adjustment is applied to the fee. Both of that makes calculating the costs for one weight very brittle. Contracts should therefore pass in higher amounts of gas to get a useful result.

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[Robbepop]

LGTM on a high-level

Eager to implement on ink! side soon.

[pepyakin]

bot merge

[ghost]

Trying merge.