Add a formatting specifier to limit the max length displayed of collections

Initially copy/pasted from Add a formatting specifier to limit the max length displayed of collections · Issue #906 · rust-lang/libs-team · GitHub, but with some minor changes. Sometimes, collections get super long, and all that really matters is the length and/or first few entries. This especially comes up in scientific analysis code and in big data processing utilities, where even test programs sometimes generate so much temporary data that a single format!("{:?}", value) could fill up mult...