When parsing, identify when decor is "default"

[epage]

This would

• Reduce allocations for toml_edit::Document
• Allow easier editing as users can make changes and the formatting "auto-adapts" (see rust-lang/cargo#12837)

This comes at the cost of

• Making it harder for callers to inspect the actual decor
• Some callers may not want to auto-adapt

These could be worked around by offering a way to force allocations for "default" decor.