oas3: v0.15.0

• Add new crate feature yaml-spec (on-by-default).
• Add top-level from_json() function.
• Add top-level from_yaml() function, guarded by the yaml-spec crate feature.
• The top-level to_json() function now returns serde_json errors directly.
• The top-level to_yaml() function now returns serde_yaml errors directly.
• The top-level to_yaml() function is now guarded by the yaml-spec crate feature.
• Remove top-level from_reader() function.
• Remove top-level from_path() function.
• Remove top-level Error type.