No-std cleanup

[ethanfrey]

Follow up from #41

• Enable the api.rs tests in no-std mode as well
• Remove #![allow(dead_code)] and #![allow(unused_imports)] from lib.rs if possible
• Fix clippy warnings and test in CI (currently compress.rs:10 recommends using matches!... but maybe only as I use 1.51 locally and we run with 1.42 in CI... let's stick with 1.42 compatibility)
• Ensure ics23.rs can be regenerated via cargo protoc without breaking the build (Fix cargo protoc error #44)

[ethanfrey]

I fixed some of this in #43

I know little about no-std, but would be happy for someone to fix the rest.