Murisi/parameterized tx2

[murisi]

An attempt to move functionality from tx.rs, rpc.rs, and signing.rs by means of parameterization by concrete clients. More concretely, the following changes were made:

• Parameterized the tx.rs and rpc.rs argument types so that they can be used with or without a Context
• Separated the pure from the impure part of Wallet and moved the pure part into the shared crate
• Moved the function implementations in rpc.rs and tx.rs into the shared crate leaving behind function wrappers in apps (that could then be used to capture Results and print them out in the CLI)
• Simplified the ShieldedContext trait by removing those functions in it that have been moved into the shared crate

With this approach, for each platform ShieldedContext, WalletUtils, and just the perform function of Tendermint's Client trait would have to be implemented (since the rest of Client has default implementations specified in terms of perform) for each client.

The only dependencies added to the shared crate are toml, bimap, orion, tokio, and serde. Hopefully this should not cause issues when building WASM targets.

[mariari]

Superseded by #921