feat: add async setup utility functions

[mattsse]

Motivation

This adds async helper functions for compiling contracts and setting up geth or ganache.
Using the async compile_and_launch_ganache will only take as long as the longest running command (solc, ganache-cli ).
This will shave off the solidity compile time for the updated examples, because this in those cases the compile time is very short. And it will shave off the ~1.8s it takes to launch ganache for those cases where solc takes longer than that.

Solution

Feature gate a setup module in ethers-core that simply runs tokio::task::blocking for the commands (solc, geth, ganache) and join those task when using compile_and_launch_ganache or compile_and_launch_geth.
By default the setup feature of the ethers-core crate is enabled in ethers exposing those helper functions from the ethers::util module.

[gakonst]

LGTM! This should become more useful once/when somebody decides to build a full dev framework on top of ethers-rs, so that it compiles the code + launches the client!