Refactor SubnetID and add ChainID to root

[adlrocha]

Addresses #78

Our previous implementation of SubnetID had two main issues:

• As we used strings to internal represent the parent of a subnet, it was dependent of the network type prefix t/f which meant that setting a different network type in Lotus and the actors led to really annoying panics/errors.
• We didn't have a way to discern among different IPC trees. For this, instead of using root to identify the root of an IPC hierarchy, we use the ChainID to "uniquely" identify different IPC trees.

Finally, we've decided that instead of supporting Ethereum addressed for subnet actors, these will be included in the path of a SubnetID through their corresponding f4 address (thus supporting any future alternative addressing that comes to the network).

Next

• Propagate the changes to the Golang implementation: refactor subnet ID go-ipc-types#21
• Integrate in Lotus/Eudico: Refactor subnet ID and infer ChainID from SubnetID lotus#187
• Assign different ChainIDs to the different subnets lotus#178
• https://github.com/consensus-shipyard/ipc-agent/issues/214

[aakoshh]

Ah cool, I didn't know there was a hasher that output u64 directly. I'll switch to using this then.

[adlrocha]

Is a non-cryptographic hash. Really convenient for this cases (and also supported by Go, so even better).

[aakoshh]

It would be good to include some examples in the form of tests. For example https://github.com/tjwebb/fnv-plus has a default bit length of 52, but also has named methods for 32 and 64. It would help users to want to produce the same chain ID on different platforms to include some documentation on what the Rust version uses.