Maximise reusability of APIs

[Wodann]

While upgrading I ran into a lot of problems using prior APIs because a lot of APIs no longer expect individual pieces of data but rather receive the "wrapping" CTX type.

To increase reusability, I believe that - where possible - APIs should use the low-level types, rather than the Context generic type. E.g.

pub fn validate_initial_tx_gas<CTX, Error>(
	context: CTX,
	spec_id: SpecId,
) -> Result<InitialAndFloorGas, Error>
where
	CTX: TransactionGetter + CfgGetter,
	Error: From<InvalidTransaction>,
{
	// implementation
}

limits the reusability for locations where you don't have a context. By changing to:

pub fn validate_initial_tx_gas<ConfigT, ErrorT, TransactionT>(
	config: &ConfigT,
	transaction: &TransactionT,
) -> Result<InitialAndFloorGas, ErrorT>
where
	ConfigT: Cfg,
	TransactionT: Transaction,

we can call it when we have a context (validate_initial_tx_gas(context.cfg(), context.tx()) while also being able to call it when we just have a transaction and config type.

For convenience, you can provide helper methods that receive the context and internally call the low-level API that's more reusable.

[rakita]

We could use even simpler fields:

pub fn validate_initial_tx_gas<Error>(
	tx: &impl Transaction,
	spec_id: SpecId,
) -> Result<InitialAndFloorGas, Error>
	Error: From<InvalidTransaction>,
{
	// implementation
}

[Wodann]

I have a local commit that does this for that function. I'll submit a PR