add negative exponential support to BigDecimal (#10892)

Co-authored-by: Johannes Müller <[email protected]>

spec/std/big/big_decimal_spec.cr

@@ -212,6 +212,12 @@ describe BigDecimal do
     result.to_s.should eq("286138.1721051424")
   end
 
+  it "exponentiates with negative powers" do
+    result = "2.0".to_big_d ** -1
+    result.should be_a(BigDecimal)
+    result.to_s.should eq("0.5")
+  end
+
   it "can be converted from other types" do
     1.to_big_d.should eq (BigDecimal.new(1))
     "1.5".to_big_d.should eq (BigDecimal.new(15, 1))

src/big/big_decimal.cr

@@ -292,9 +292,7 @@ struct BigDecimal < Number
   # BigDecimal.new(1234, 2) ** 2 # => 152.2756
   # ```
   def **(other : Int) : BigDecimal
-    if other < 0
-      raise ArgumentError.new("Negative exponent isn't supported")
-    end
+    return (to_big_r ** other).to_big_d if other < 0
     BigDecimal.new(@value ** other, @scale * other)
   end