Generalise constructor blocks into inline block expressions

[Tamschi]

Currently, a special constructor like the following is available once:

new with {
    let _ = local_resource_node.borrow();
}

This is not ideal, as it is rarely-used special syntax and unlike with { … } … expressions can not be used arbitrarily on subexpressions.

A possible change would be to to generalise and unify these blocks and expressions into *inline "Rust" { … } … and .inline "Rust" { … } … expressions, for the constructor and render scope respectively. The constructor-scoped inline blocks should support dependency injection in a scoped manner (though how to make this flow nicely to content children is an unsolved problem. It may still be more clear to still require using additional, transparent container components for this purpose).

Iff scoped dependency injection is allowed in some way in the future, the branch-on-borrow handle should be renamed from local_resource_node to scoped_resource_node.