Fix linting issues

[ethanfrey]

Part of #42

• Add cargo fmt and cargo clippy to CI
• Fix fmt issues
• Fix clippy issues
• Remove #![allow(dead_code)] and #![allow(unused_imports)] from lib.rs

[ethanfrey]

@DaviRain-Su can you please review this.

While trying to address some of the clippy warnings (and running 1.42 locally to get the same as the CI), I realized the ics23.rs file was manually editted and doesn't match what comes from cargo protoc, which will cause issues in the future keeping them in sync. I tried regenerating it with the script and got all kinds of errors, even in std mode.

Otherwise the rest was just making clippy happy basically