Document fragments in template! macro

[lukechu10]

Describe the solution you'd like
The template! macro should allow document fragments without the <></> JSX syntax.
The following should be accepted by the template! macro:

// multiple children
template! {
    p { ... }
    a { ... }
};

// empty fragment
template! {}

The TemplateResult struct should be converted into an enum with the variants Single and List where the second variant is used for document fragments (including empty fragments). This should not use the DocumentFragment under the hood as it behaves somewhat differently from Nodes. For one thing, when appending a DocumentFragment to the DOM, the elements are moved out of the fragment and no longer accessible. Instead, TemplateResult::List should use a Vec or other collection under the hood.

Questionnaire

• I would like to implement this feature but I don't know where to start
• I would like to implement this feature and I have a solution
• I don't have time to implement this / I don't want to implement this

Edit
The current way Keyed and Indexed are a bit buggy. They return DocumentFragments which, as described above, have certain differences from other html nodes. This results in a panic when updating Keyed and Indexed at the top level of a template!, e.g.

template! {
    Keyed {..} // panics when iterable is updated
}