Use yield instead of block.call

Reverts the change motivated by https://bugs.ruby-lang.org/issues/11451,
see also #189.

The underlying issue was fixed in r52717, which is in Ruby
2.2.5. This requires some discussion about whether support for earlier
Ruby 2.2.x releases can be dropped at this stage.

lib/hamster/trie.rb

@@ -29,15 +29,7 @@ def key?(key)
 
     # Calls <tt>block</tt> once for each entry in the trie, passing the key-value pair as parameters.
     def each(&block)
-      # TODO: Using block.call here is slower than using yield by 5-10%, but
-      # the latter segfaults on ruby 2.2 and above. Once that is fixed and
-      # broken versions are sufficiently old, we should revert back to yield
-      # with a warning that the broken versions are unsupported.
-      #
-      # For more context:
-      # * https://bugs.ruby-lang.org/issues/11451
-      # * https://github.com/hamstergem/hamster/issues/189
-      @entries.each { |entry| block.call(entry) if entry }
+      @entries.each { |entry| yield(entry) if entry }
       @children.each do |child|
         child.each(&block) if child
       end