Time-Aware Base Fee Calculation

[adietrichs]

wip

todo:

• adapt specification
• ensure all low-number edge cases are covered
• add MAX_GAS_TARGET_PERCENT to rationale
• write out motivation, rationale, backward compatibility, security considerations (so far: keywords)
• write abstract

[mkalinin]

I would suggest to be explicit about what exactly has been changed comparing to the original formula. Also, it would probably be clearer to have a function compute_parent_adjusted_gas_target and replace parent_gas_target in the original formula with parent_adjusted_gas_target = compute_parent_adjusted_gas_target(...)