Add BigDecimal.with_rounding_mode

spec/std/big/big_decimal_spec.cr

@@ -277,6 +277,14 @@ describe BigDecimal do
     end
   end
 
+  describe ".with_rounding_mode" do
+    it "sets given mode as .rounding_mode within yielded block" do
+      BigDecimal.with_rounding_mode(:half_even) do
+        BigDecimal.rounding_mode.should eq(BigDecimal::RoundingMode::HALF_EVEN)
+      end
+    end
+  end
+
   it "performs arithmetic with other number types" do
     (1.to_big_d + 2).should eq(BigDecimal.new("3.0"))
     (2 + 1.to_big_d).should eq(BigDecimal.new("3.0"))

src/big/big_decimal.cr

@@ -286,6 +286,28 @@ struct BigDecimal < Number
     end
   end
 
+  # Executes the provided block, preserving the rounding mode; *mode* argument
+  # is set as `BigDecimal.rounding_mode` inside the block.
+  #
+  # ```
+  # BigDecimal.rounding_mode = :half_even
+  #
+  # # *mode* argument is set as `BigDecimal.rounding_mode` inside the block
+  # BigDecimal.with_rounding_mode(:up) do
+  #   BigDecimal.rounding_mode # => BigDecimal::RoundingMode::UP
+  # end
+  #
+  # BigDecimal.rounding_mode # => BigDecimal::RoundingMode::HALF_EVEN
+  # ```
+  #
+  # NOTE: Uses `save_rounding_mode` internally.
+  def self.with_rounding_mode(mode : RoundingMode?)
+    save_rounding_mode do
+      @@rounding_mode = mode
+      yield
+    end
+  end
+
   # Rounds to the nearest integer (by default), returning the result as a `BigDecimal`.
   #
   # ```