Also fix capacity of Poolable for HashMap<T>

It seems the hashmap capacity is, similarly to Vec, not exactly
controllable and we can not always guarantee len == capacity.  Again
similarly when Block::size() returns a given lenght you expect to be
able to index to this length.

Author: flub

src/poolable.rs

@@ -23,7 +23,7 @@ where
     S: BuildHasher + Default,
 {
     fn capacity(&self) -> usize {
-        self.capacity()
+        self.len()
     }
 
     fn alloc(size: usize) -> Self {