ensure that IsCancun is used as much as possible

[gballet]

In many instances, evm.txContext.Accesses != nil can be replaced by a call to IsCancun(), we should strive for using this, in order to make it easier for reviewers to figure out that this code will not activate outside of a verkle tree-enabled context, even if Accesses are somehow activated.

[gballet]

@jwasinger did most of the work, there are a couple issues left in PR gballet/go-ethereum#60:

core/vm/evm.go:	if evm.Accesses != nil && value.Sign() != 0 {
core/vm/evm.go:	if err == nil && evm.Accesses != nil {

[gballet]

Solved with 4087d8b