feat: Instantiate `Context` for Starknet types

[romac]

• Add a new crate malachite-starknet to the repository
• Figure out where to find required types for Starknet
• Instantiate the Context with those types
• Adapt Context trait and associated interfaces to make it possible to run consensus using these concrete types

[adizere]

These types? https://github.com/starknet-io/types-rs

[romac]

Afaict this repo only contains definition for the Felt type, but not any of the types we are interested in.