Manta Core Libraries API Redesign

[bhgomes]

Goals

1. Combine the existing Manta Rust libraries into one project.
2. Implement the current specifications with no extra implementation details or dependencies (i.e. try to stick to only the business logic, so for example, there should be no substrate/ethereum dependencies, those should be entirely part of pallet-manta-pay, pallet-manta-swap, Manta-ERC20, ... etc.).
3. Improve developer experience including full utilization of GitHub project management tools, automated workflows, and increased repo security. See the Projects tab for more on this effort which will extend beyond this particular PR.

Structure

The new project replaces the existing Manta Rust libraries with library crates in a shared workspace:

• manta-util: some Rust utilities
• manta-crypto: abstract cryptographic primitive definitions
• manta-accounting: accounting primitives, including address system, transfer protocol, and abstract wallet implementation
• manta-pay: concrete implementation of the MantaPay spec using manta-crypto and manta-accounting primitives