core: add support for boxed Collect
objects
#1371
Closed
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This adds
impl Collect for Box<dyn Collect + Send + Sync + 'static>
which can make it easier to work with collectors that have a type that
is hard to name.
That tends to happen when building collectors that have been composed
with lots of layers and you want to return them from functions or take
them as arguments. Using a trait object makes that easier.