Refactor `goto` statements in stream.cpp's `encodeutf8()`

[no-lex]

https://github.com/project-imprimis/imprimis/blob/17eb9bc56cf668d8969a6bc3a3ecdacb4721d00f/src/shared/stream.cpp#L184-L226

This function uses a lot of goto statements which makes it hard to understand. Most of these should be axed, or failing that, well commented.