Specifically, remove it in favor of making the internal value_utf8_lossy into a public API, which by itself will cover the need for working with a document with possibly-corrupted string values.

When we do this, we will need to revisit whether value_utf8_lossy should return:

• An enum with a restricted set of values as it currently does, or a full Bson enum
• Always-owned values as it currently does, or some flavor of CoW to avoid copying when no invalid utf8 is found.