Run tests under Miri

[michaelsproul]

• Update ethereum_hashing to allow disable ring (done in v0.8)
• Update tree_hash to depend on v0.8 of hashing, and cut a new BREAKING release (v0.11)
• Update ssz_types to depend on the new tree_hash version
• Run test suite for ssz_types under Miri (maybe we pin a nightly version of the compiler)

This will enable us to merge:

• Optimise SSZ encoding and decoding #55

[macladson]

New tree_hash release pending -> sigp/tree_hash#37