radicle: Implement Iterator::size_hint() for optimizations

This patch adds an implementation of the Iterator::size_hint() trait
function so that the compiler can optimize allocations on the collecting
side of the iterator better.

From the trait function docs:

> size_hint() is primarily intended to be used for optimizations such
> as reserving space for the elements of the iterator, but must not be
> trusted to e.g., omit bounds checks in unsafe code. An incorrect
> implementation of size_hint() should not lead to memory safety
> violations.

Source: https://doc.rust-lang.org/stable/std/iter/trait.Iterator.html#method.size_hint

And:

> Vec may use any or none of the following strategies, depending on the
> supplied iterator:
>
>  *  preallocate based on Iterator::size_hint() and panic if the number
>     of items is outside the provided lower/upper bounds

Source: https://doc.rust-lang.org/stable/std/iter/trait.FromIterator.html

Signed-off-by: Matthias Beyer <mail@beyermatthias.de>

crates/radicle/src/cob/issue/cache.rs

@@ -371,6 +371,10 @@ impl Iterator for NoCacheIter<'_> {
     fn next(&mut self) -> Option<Self::Item> {
         self.inner.next()
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.inner.size_hint()
+    }
 }
 
 impl<R> Issues for Cache<super::Issues<'_, R>, cache::NoCache>
@@ -449,6 +453,10 @@ impl Iterator for IssuesIter<'_> {
         let row = self.inner.next()?;
         Some(row.map_err(Error::from).and_then(IssuesIter::parse_row))
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.inner.size_hint()
+    }
 }
 
 impl<R> Issues for Cache<R, StoreWriter>