bellman: remove deprecated and unmaintained futures_cpupool

[hdevalence]

futures_cpupool uses the old-style 0.1 futures that are no longer actively maintained and are not interoperable with the rest of the rust futures ecosystem. It should be removed from bellman.

One option is to continue to use std::future::Future futures. These require an executor. One option is the threadpool from the futures crate, a simple executor which just handles CPU-bound work. Another option is Tokio's blocking threadpool.

Another option would be to deprecate the use of futures and use a synchronous API instead.

Not knowing more about the design of bellman it's hard for me to determine which of the two high-level options is the best.

[hdevalence]

Perhaps another option to consider is dropping the multicore / non-multi-core distinction in the bellman code, using Tokio's spawn_blocking unconditionally, and outsourcing the single/multi-threading configuration to the configuration of the Tokio runtime.

[str4d]

Closed by zkcrypto/bellman#69 (which backported the Filecoin changes to migrate bellman to rayon).