Feature request: hide most warnings by default

[Nadrieril]

It is a common experience when running Charon on a new crate to get a massive wall of warnings. Most of these are useless to the user. I propose that we:

• Hide the text of warnings by default;
• Display something like "X warnings emitted, the crate could not be translated fully";
• Add a --show-warnings flag to restore the old output (and enable it by default in tests);
• Add has_errors: bool to ItemMeta to track which items have errors.