feat: POC of contracts pallet

[sander2]

This adds pallet_contracts to the standalone runtime. I added a contract in contracts/hello_world that has a function that interacts with the runtime, and a test in standalone/runtime/tests/test_contracts.rs to show that it's working. The test is simple: the contract calls a function in the runtime that returns some value (in this case, it returns Alice's balance), followed by another call that sets some value. This way, we test that (1) passing arguments to runtime functions work, (2) those functions can return values, and (3) that these functions can have side effects (storage changes). The implementation of the runtime side of these functions can be found in standalone/runtime/src/lib.rs

To test for yourself:

• run cargo install cargo-contract
• cd to the contracts/hello_world folder and run cargo contract build
• open standalone/runtime/tests/test_contracts.rs
  • Modify the path in test_contract
  • run test_contract
  • Observe that the test passes

ps don't mind the dex changes - they are unrelated

Closes #983

[gregdhill]

Succeeded by #1114