Fixing clippy warnings

[Angel-Petrov]

This PR fixes clippy warnings which were most easily able to be fixed.

The primary chunk of warnings were due to a associated function Self::default() existing but also implementing Default. There should (hopefully) be no breaking changes from removing the associated function as user's code will fall back to Default::default.

Another clippy warning fix that is a potential breaking change is implementing Default on Transaction and Tx, though this is only a issue in contrived scenarios and the crate is still in 0.x.

A weird edge case was when encountering Config::default_main(), I decided to keep the associated function for stability reasons and I just made Default::default() call it.

As for the remaining clippy warnings, only 3 remain:

• Associated function new doesn't return Self for many types
  • I am not familiar enough with the codebase to see where these functions should be moved to
• Function has too arguments for CommonVm::initialize
• Module has the same name as its containing module for proto/pb/message.rs
  • Since I think this is intentional, maybe it should be turned off? Or maybe the module can be removed.

[richardpringle]

Wow... I've done this before and it was not a fun exercise. Thank you!

[richardpringle]

@Angel-Petrov, just need to run cargo fmt to fix this (and pull in any updates)