gas SBAT variably-charged based off of exponent in fracMulExp precompile

[nityas]

Expected Behavior

we current charge a flat rate(1050) that covers up to an exponent of 10^3, this should vary with the log of the exponent

this also needs to be updated in the following places:

• fc-fractionmulexp.js in ethereumjs-vm
• fractionMulExp in StableToken.sol

Current Behavior

fixed gas cost

[prestwich]

I vote to close in favor of adopting EIP-2565.

edit: updating this off the cuff response :)

I am unclear on the use case, but it seems we can always compute something simpler than fracMulExp using modExp