Add uuid.Nil and .IsZero() method

This simplifies comparisons with empty ULIDs.  Right now, most people
are doing something like the following to check if a ULID is non-zero:

```go
id.Compare(ulid.ULID{}) == 0
```

This requires allocating a new empty ULID each time.  Some packages
avoid this by creating their own global (or private) nil ULIDs on
initialization.

This should be built in and simple for better perf & a cleaner API.

ulid.go

@@ -75,6 +75,9 @@ var (
 	// ErrScanValue is returned when the value passed to scan cannot be unmarshaled
 	// into the ULID.
 	ErrScanValue = errors.New("ulid: source value must be a string or byte slice")
+
+	// Nil is an empty ULID, all zeros, useful when comparing nil ULIDs
+	Nil ULID
 )
 
 // MonotonicReader is an interface that should yield monotonically increasing
@@ -416,6 +419,11 @@ func (id ULID) Timestamp() time.Time {
 	return Time(id.Time())
 }
 
+// IsZero returns whether the ULID is a zero-value, ie ulid.Nil.
+func (id ULID) IsZero() bool {
+	return bytes.Compare(id[:], Nil[:]) == 0
+}
+
 // maxTime is the maximum Unix time in milliseconds that can be
 // represented in a ULID.
 var maxTime = ULID{0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF}.Time()

ulid_test.go

@@ -454,6 +454,20 @@ func TestULIDTimestamp(t *testing.T) {
 	}
 }
 
+func TestZero(t *testing.T) {
+	t.Parallel()
+
+	var id ulid.ULID
+	if ok := id.IsZero(); !ok {
+		t.Error(".IsZero: must return true for empty ULIDs, have false")
+	}
+
+	id = ulid.MustNew(ulid.Now(), ulid.DefaultEntropy())
+	if ok := id.IsZero(); ok {
+		t.Error(".IsZero: must return false for non-nil ULIDs, have true")
+	}
+}
+
 func TestEntropy(t *testing.T) {
 	t.Parallel()