This updates the hardhat task `genesis-l2` to handle
setting the immutables in the deployed bytecode. This
is necessary because the predeploys "live" in the L2
state and are not deployed. Any immutables must be manually
placed into the bytecode by looking at the compiler output
and then finding the offsets and slicing in the values.

A high level API is exposed for setting arbitrary immutables
with the `replaceImmutables` function. This was inspired by
smock's `computeStorageSlots`.

Longer term, this code will be rewritten in go and be able to
operate on either a LevelDB database (upgrade an existing system)
or a `genesis.json` (good for starting a new network). This is
a requirement because the state surgery is large enough such that
it must bind directly to LevelDB and be in a language that is
relatively fast.