Add support for 32GiB sectors to devnet

[nonsense]

We should add support for 32GiB sectors to the devnet in a way that allows developers to iterate on retrieval codebase without too much overhead.

Specifically this means:

• add support for 32GiB sectors
• create a genesis block with 2 presealed 32GiB sectors -- include a marker file that detects if this step has been run on developers' machine
• create a predefined set of car files that a developer can easily store and retrieve in a fast feedback loop, similar to the autodeal tool -- https://github.com/nonsense/autodeal/blob/master/spx-notes.json
• add a marker file around the lotus fetch-params 34359738368 in order to speed up devnet startup

[jennijuju]

You probably need actor changes for it

[jennijuju]

https://github.com/filecoin-project/builtin-actors/blob/6df845dcdf9872beb6e871205eb34dcc8f7550b5/runtime/build.rs#L16

[nonsense]

@jennijuju I was hoping we can use testing-fake-proofs instead of devnet.