could GasPolicy have more detail abount op

[hunjixin]

could GasPolicy have more detail abount op like below

type GasPolicy interface {
	GetCost(key Instr) int64
}

some time more detail may be needed
eg:
GrowMemory op. i want to know how many pages to grow, op gas may be n* pageGas.

if think it ok. i can have a pr

[losfair]

Yeah I think this is a good idea!

Thanks for the suggestion and a PR is welcome :)