Fix protect Array#map against concurrent mutation

src/array.cr

@@ -1113,7 +1113,9 @@ class Array(T)
 
   # Optimized version of `Enumerable#map`.
   def map(& : T -> U) : Array(U) forall U
-    Array(U).new(size) { |i| yield @buffer[i] }
+    map_with_index do |item, _|
+      yield item
+    end
   end
 
   # Modifies `self`, keeping only the elements in the collection for which the
@@ -1216,7 +1218,14 @@ class Array(T)
   # results # => ["0: crystal", "1: pearl", "2: diamond"]
   # ```
   def map_with_index(offset = 0, & : T, Int32 -> _)
-    Array.new(size) { |i| yield @buffer[i], offset + i }
+    Array(U).build(size) do |build_buffer|
+      i = 0
+      while i < size
+        build_buffer[i] = yield @buffer[i], offset + i
+        i &+= 1
+      end
+      i
+    end
   end
 
   # Returns an `Array` with the first *count* elements removed