Unmarshalling float with lots of digits can cause >= 2 allocations per number

We can fix this with a little bit of scratch space.

benchmarks/float_test.go

@@ -0,0 +1,28 @@
+package test
+
+import (
+	"testing"
+
+	jsoniter "github.com/json-iterator/go"
+)
+
+func BenchmarkFloatUnmarshal(b *testing.B) {
+	type floaty struct {
+		A float32
+		B float64
+	}
+
+	data, err := jsoniter.Marshal(floaty{A: 1.111111111111111, B: 1.11111111111111})
+	if err != nil {
+		b.Fatal(err)
+	}
+
+	b.ResetTimer()
+	b.ReportAllocs()
+	var out floaty
+	for i := 0; i < b.N; i++ {
+		if err := jsoniter.Unmarshal(data, &out); err != nil {
+			b.Fatal(err)
+		}
+	}
+}