Similar to native match pattern for braced structs, make matches_pattern enforce exhaustive field checks for braced structs by default, to be suppressed by explicit .. at the end of the pattern.

[copybara-service]

Similar to native match pattern for braced structs, make matches_pattern enforce exhaustive field checks for braced structs by default, to be suppressed by explicit .. at the end of the pattern.

This change is not backward-compatible since:

• It enforces field exhaustiveness by default in the absence of a trailing .. in the pattern.
• Things that previously fell back to match pattern matching now use match_pattern!'s specialized matching, which requires the matched object to implement Debug.

Toward #447