Added Sequence.each

This is a bit of a style preference since of course you can always write
the same thing with a for loop. However, I think sometimes the code
looks better when using this method.

It also provides an alternative to Sequence.map for cases where you
don't need the resulting list, and one that becomes especially necessary
when Sequence.map is changed to return a new sequence. The example in
the README.md file was using Sequence.map in a way that required this
alternative in that case.

README.md

@@ -13,7 +13,7 @@ class Wren {
 }
 
 var adjectives = new Fiber {
-  ["small", "clean", "fast"].map {|word| Fiber.yield(word) }
+  ["small", "clean", "fast"].each {|word| Fiber.yield(word) }
 }
 
 while (!adjectives.isDone) IO.print(adjectives.call())