Update ethereum-types to 0.5

[michaelsproul]

Issue Addressed

Update to ethereum-types 0.5.0 #216

Proposed Changes

• Update ethereum-types to 0.5(.2)
• Use from_slice to convert byte slices to Hash256, as Hash256::from for slices has been removed
• Add explicit as_bytes conversions for Hash256 to &[u8]. Previously not required due to a now-removed Deref impl
• Use int_to_bytes32 in generate_seed to avoid confusion (spec)
• Replace the arbitrary strings used for the epoch seeds in teleport_to_end_of_epoch with arbitrary byte arrays
• Add a Hash256::from_short_slice mixin to ease compatibility with existing tests that use slices of length less than 32

Additional Info

I'd like feedback on the last two points in particular, which feel a little bit suspect – particularly from_short_slice. We could do away with that extra complexity by using arbitrary byte arrays in the tests instead of strings like "some hash".

[paulhauner]

Looks great, thanks!

WRT the last two points:

• Replacing the teleport_to_end_of_epoch with other arbitrary bytes is fine. They're truly arbitrary at this stage.
• I don't have a string opinion about from_short_slice. My intuition is to replace the "some_hash" with vec![42; 32] or something similar. Hash256::random(..) could also work -- although potentially harder to troubleshoot in an assertion-failed error message.

Happy to go with whatever you prefer for from_short_slice(..).

[michaelsproul]

from_short_slice is a hack to remedy a hack, I'll remove it and replace the strings by byte arrays :)

[paulhauner]

LGTM!