Fix an age-old bug with FromFloat handling of negatives. (#268)

* Fix an age-old bug with `FromFloat` handling of negatives.

 * (M) pkg/yang/types_builtin(_test)?.go
    - This change squashes a bug that has existed for a long time
      in `FromFloat`, but only shows up on arm64. I finally have an
      arm64 machine and so found the issue. It was originally reported
      in openconfig/ygot#766. The issue is
      that uint64 of a negative float64 is undefined  -- and on arm64
      on darwin returns 0, which made some test cases fail only on this
      architecture.

* Add missing comment.

* `gofmt`

pkg/yang/types_builtin.go

@@ -334,12 +334,12 @@ func FromFloat(f float64) Number {
 	for ; Frac(f) != 0.0 && fracDig <= MaxFractionDigits; fracDig++ {
 		f *= 10.0
 	}
-	v := uint64(f)
 	negative := false
 	if f < 0 {
 		negative = true
-		v = -v
+		f = -f
 	}
+	v := uint64(f)
 
 	return Number{Negative: negative, Value: v, FractionDigits: fracDig}
 }

pkg/yang/types_builtin_test.go

@@ -55,6 +55,54 @@ func Rf(a, b int64, fracDig uint8) YRange {
 	return YRange{n1, n2}
 }
 
+func TestFromFloat(t *testing.T) {
+	tests := []struct {
+		desc string
+		in   float64
+		want Number
+	}{{
+		desc: "positive - no decimals",
+		in:   10.0,
+		want: Number{
+			Negative:       false,
+			Value:          10,
+			FractionDigits: 0,
+		},
+	}, {
+		desc: "positive - decimals",
+		in:   10.15,
+		want: Number{
+			Negative:       false,
+			Value:          1015,
+			FractionDigits: 2,
+		},
+	}, {
+		desc: "negative - no decimals",
+		in:   -10.0,
+		want: Number{
+			Negative:       true,
+			Value:          10,
+			FractionDigits: 0,
+		},
+	}, {
+		desc: "negative - decimals",
+		in:   -10.15,
+		want: Number{
+			Negative:       true,
+			Value:          1015,
+			FractionDigits: 2,
+		},
+	}}
+
+	for _, tt := range tests {
+		t.Run(tt.desc, func(t *testing.T) {
+			if got := FromFloat(tt.in); !cmp.Equal(got, tt.want) {
+				t.Fatalf("FromFloat(%v): did not get expected value, got: %+v, want: %+v", tt.in, got, tt.want)
+			}
+		})
+	}
+}
+
 func TestNumberInt(t *testing.T) {
 	tests := []struct {
 		desc    string