Some of the other clients don't bother to store block numbers as
256-bit integers (64 bits or even 32 bit are enough).
The changes here are based on helpers introduced in eth_common
that try to abstract away all conversion steps in the code between
VM words and block numbers in variables in fields. Unfortunately,
this turn out to be more work than anticipated and I've postponed
finishing the refactoring for now (Block numbers are still 256 bit).
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