Crate features to switch between functionality

[Zk2u]

HardLight is an all-in-one package currently. This includes a multithreaded RPC server using Tokio and macros and helpers for developers using HardLight.

Some dependencies for certain functionalities should be shipped in a different build. For example, the WebAssembly client should NOT be shipping tokio and a multithreaded async runtime to the browser or the other server-specific dependencies/features.

We can separate these by using "features" in the crate to allow users to enable/disable parts of the crate based on where and what they're shipping.

This shouldn't be too hard, mostly meta stuff in the Cargo.toml and enabling/disabling code based on that.