## Why this should be merged
The ability to deterministically generate private keys (and
corresponding EOAs) is useful in testing.
## How this works
A `[]byte` seed is written to a Keccak state, which is rejection-sampled
to find a scalar `<` the order of `S256` to use as the private key.
## How this was tested
Unit test that demonstrates (a) determinism of generation; and (b)
correct computation of the public key as proven by ECDSA recovery using
the standard tx-sender functionality.
