Speed up gas metering by batching instead of per-instruction

[daiagi]

What we need to do

Right now every single instruction calls gas_cost() and creates a new state struct. Since all instructions cost 1 gas anyway, we can batch this and only check gas every 16 instructions or so.

Expected outcome

15-20% speedup by eliminating overhead on every instruction

Code example

Current (slow):

# This happens for EVERY instruction
gas_cost = Instructions.gas_cost(opcode)  # Always returns 1
deduct_gas = %{state_ | gas: state.gas - gas_cost}

Better (fast):

# Only check gas every 16 instructions
instruction_count = instruction_count + 1
if rem(instruction_count, 16) == 0 do
  new_state = %{state_ | gas: state.gas - 16}
  if new_state.gas < 0, do: {:out_of_gas, new_state}
end

This eliminates the pattern matching and struct creation on 15 out of every 16 instructions.