perf(interpreter): keep track of remaining gas rather than spent

[DaniPopes]

Removes a load and a comparison in the very hot record_cost function (see diff below) by using overflowing_sub instead of saturating_add + < limit.

Reduces snailtracer instructions from 2,768,629,456 to 2,661,673,460 (-3.86%).

I've also renamed Gas::spend to Gas::spent (with a deprecation warning).

[mattsse]

I think tracking remaining gas is more intuitive.

afict this merely flips cost recording logic

[rakita]

This is cool! We dont need to compare it with gas.limit anymore