Implement `From<inner type>` for `Item`

[pickx]

one thing I found surprising while working with toml_edit after using toml, is that Item doesn't implement From<{Value,Table,ArrayOfTables}>, in contrast with toml::Value which does have equivalent From implementations. this makes code that manually builds a document a little more verbose.

and maybe there are more things in toml_edit which could benefit from a From implementation, like toml_edit::Value or Formatted.