Unifications for compatibility with go-lachesis

[Maxime2]

Current Lachesis implementation in Go uses FNV hash function to calculate EventHash, https://golang.org/pkg/hash/fnv/ ; but Rust has replaced FNV hash as its default hash function by a faster hash function, rust-lang/rust#37229

To make lachesis-rs compatible with go implementation we need to use the very same hash function for event hashes. See https://github.com/servo/rust-fnv for fnv hash implementation in Rust.

[Maxime2]

Actually EventHash is calculated as SHA256(serialised event body); and go-lachesis and lachesis-rs have different event body definitions and different serialisation methods, - these ones need to be unified among all implementations of Lachesis, at least for things sent between nodes.