Support BigFloat#** for all Int::Primitive arguments (#13971)

Author: HertzDevil

spec/std/big/big_float_spec.cr

@@ -186,6 +186,9 @@ describe "BigFloat" do
     it { ("-0.05".to_big_f ** 10.to_big_i).should be_close("0.00000000000009765625".to_big_f, 1e-12) }
     it { ("0.1234567890".to_big_f ** 3.to_big_i).should be_close("0.001881676371789154860897069".to_big_f, 1e-12) }
 
+    it { ("10".to_big_f ** -5).should be_close("0.00001".to_big_f, 1e-12) }
+    it { ("0.1".to_big_f ** -5).should be_close("100000".to_big_f, 1e-12) }
+
     it { ("10".to_big_f ** (-5).to_big_i).should be_close("0.00001".to_big_f, 1e-12) }
     it { ("0.1".to_big_f ** (-5).to_big_i).should be_close("100000".to_big_f, 1e-12) }
     it { ("0".to_big_f ** 1.to_big_i).should eq(0.to_big_f) }

src/big/big_float.cr

@@ -171,7 +171,18 @@ struct BigFloat < Float
   end
 
   def **(other : Int) : BigFloat
-    BigFloat.new { |mpf| LibGMP.mpf_pow_ui(mpf, self, other.to_u64) }
+    # there is no BigFloat::Infinity
+    if zero? && other < 0
+      raise ArgumentError.new "Cannot raise 0 to a negative power"
+    end
+
+    Int.primitive_ui_check(other) do |ui, neg_ui, big_i|
+      {
+        ui:     BigFloat.new { |mpf| LibGMP.mpf_pow_ui(mpf, self, {{ ui }}) },
+        neg_ui: BigFloat.new { |mpf| LibGMP.mpf_pow_ui(mpf, self, {{ neg_ui }}); LibGMP.mpf_ui_div(mpf, 1, mpf) },
+        big_i:  self ** {{ big_i }},
+      }
+    end
   end
 
   def abs : BigFloat