Add new optimized version for Array#map based on Enumerable#map

src/array.cr

@@ -1111,6 +1111,13 @@ class Array(T)
     @size = size.to_i
   end
 
+  # Optimized version of `Enumerable#map`.
+  def map(& : T -> U) : Array(U) forall U
+    map_with_index do |item, _|
+      yield item
+    end
+  end
+
   # Modifies `self`, keeping only the elements in the collection for which the
   # passed block is truthy. Returns `self`.
   #
@@ -1200,6 +1207,22 @@ class Array(T)
     end
   end
 
+  # Optimized version of `Enumerable#map_with_index`.
+  #
+  # Accepts an optional *offset* parameter, which tells it to start counting
+  # from there.
+  #
+  # ```
+  # gems = ["crystal", "pearl", "diamond"]
+  # results = gems.map_with_index { |gem, i| "#{i}: #{gem}" }
+  # results # => ["0: crystal", "1: pearl", "2: diamond"]
+  # ```
+  def map_with_index(offset = 0, & : T, Int32 -> U) forall U
+    ary = Array(U).new(size)
+    each_with_index(offset) { |e, i| ary << yield e, i }
+    ary
+  end
+
   # Returns an `Array` with the first *count* elements removed
   # from the original array.
   #