* "Support" spans for maps
In toml you can declare maps via {} and via [name].
We can't obtain spans for [] maps but at least we
can emit fake spans to make SpannedValue work.
We also add a regression test.
* Don't regress the inline table case
* Also support arrays
* Store key spans in the deserializer
* Support deserializing spanned keys
* Store key spans of the table header as well
* Support nested table key spans as well
