The TOML format has a restriction that if a table itself contains
tables, all keys with non-table values must be emitted first.
This commit enforces said order by modifying value::Values recursively
before handing them to the TOML serializer.

toml 0.6 is based on toml_edit, it:
 - allows to preserve order
 - fixes some mis-serializaitons, e.g. with mixed lists
 - does not require order of non-/table values in maps

This reverts the previous commit.