problems with ethereum ethash config

[sekisamu]

because it is hard to collect all ethereum ethash-related configs, it is impossible to precisely compute the difficulty.

below is the test error:

failures:

---- pow::tests::can_calculate_difficulty stdout ----
thread 'pow::tests::can_calculate_difficulty' panicked at 'assertion failed: `(left == right)`
  left: `2585329492207444`,
 right: `2581687209681081`', core/sr-eth-primitives/src/pow.rs:480:3
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

It seems that we need more collections here, and some help from ethereum dev team.