Deduplicates some constants and ensures a single source of truth
for defining values. Helps to reduce confusion when reading the specs.
These specs are important for understanding the security properties
of the ecotone upgrade where system upgrade transactions are used
to trigger a hardfork upgrade of L2 predeploys. The invariant must
be held that it is not possible to create an L2 block that is larger
than the block gas limit when it contains all of the user deposits
plus the upgrade txs.