Function canonicalize 

pub fn canonicalize(xml: &str) -> Result<String, CanonicalizationError>

Canonicalize an XML document per RFC 3076 (Canonical XML, no comments).

Steps:

1. Strip XML declaration (<?xml ... ?>)
2. Strip processing instructions and comments
3. Normalize line endings to \n
4. Normalize attribute value whitespace
5. Expand character references (& → &, etc.)
6. Re-encode special characters in text
7. Encode as UTF-8