perf: more lightweight second gas limit

[joonazan]

The validation gas limit requires being able to limit the amount of gas that is processed. The original implementation had to scan through the whole callstack on every single instruction. This fixes that so that the scan is done at most once per call.

[slowli]

I may be missing something, but I think this logic doesn't always work. Suppose we have gas_limit = 1000 with the execution in the bootloader with 10000 gas left; i.e., initially minimum_gas = 10000 - 1000 = 9000. Suppose the bootloader calls into another contract passing 500 gas to it. Then, since 500 < minimum_gas, minimum_gas will be updated to 500.saturating_sub(1000 - spent) = 0 (assuming spent is sufficiently small). Then, self.state.current_frame.gas < minimum_gas will always be false (including after the called contract returns), so gas limiting won't work.