Fix all "cargo build" warnings and "cargo clippy" issues.

[play-tank]

Fixed all current build warnings in the code.

Tested with the u-he TyrellN6 and Diva CLAP plug-ins. I got the same results (PASSES and ERRORS) running the code before and after the changes.

There are no tests or contrib guidelines in this project, so I'm not sure what else to test.