Replace number formatting operations on Number

This replaces the implementations of toExponential, toFixed,
and toPrecision on the Number class with new implementations that use
a combination of BigDecimal and the new double formatting code.

Author: gbrail

benchmarks/src/jmh/java/org/mozilla/javascript/benchmarks/NumberFormatBenchmark.java

@@ -2,6 +2,7 @@
 
 import java.util.concurrent.TimeUnit;
 import org.mozilla.javascript.ScriptRuntime;
+import org.mozilla.javascript.dtoa.DecimalFormatter;
 import org.openjdk.jmh.annotations.Benchmark;
 import org.openjdk.jmh.annotations.OutputTimeUnit;
 
@@ -34,4 +35,29 @@ public Object scriptRuntimeSmall() {
     public Object scriptRuntimeBig() {
         return ScriptRuntime.toString(314000000000000000000.0);
     }
+
+    @Benchmark
+    public Object toFixedPi() {
+        return DecimalFormatter.toFixed(Math.PI, 20);
+    }
+
+    @Benchmark
+    public Object toExponentialPi() {
+        return DecimalFormatter.toExponential(Math.PI, 20);
+    }
+
+    @Benchmark
+    public Object toExponentialPiDefault() {
+        return DecimalFormatter.toExponential(Math.PI, -1);
+    }
+
+    @Benchmark
+    public Object toFixedDenormal() {
+        return DecimalFormatter.toFixed(DENORMAL, 20);
+    }
+
+    @Benchmark
+    public Object toExponentialDenormal() {
+        return DecimalFormatter.toExponential(DENORMAL, 20);
+    }
 }