Pull the 2006-01-02 layout to the top when checking

The current top 2 formats are, from some checks I've done, 2 dominant formats used in Hugo projects.

Checking these first has a big effect:

```bash

name                  old time/op    new time/op    delta
CommonTimeLayouts-10    1.45µs ± 0%    0.33µs ± 1%  -77.45%  (p=0.029 n=4+4)

name                  old alloc/op   new alloc/op   delta
CommonTimeLayouts-10    1.55kB ± 0%    0.18kB ± 0%  -88.14%  (p=0.029 n=4+4)

name                  old allocs/op  new allocs/op  delta
CommonTimeLayouts-10      38.0 ± 0%       6.0 ± 0%  -84.21%  (p=0.029 n=4+4)
```

See gohugoio/hugo#10942

cast_test.go

@@ -897,6 +897,17 @@ func BenchmarkTrimZeroDecimal(b *testing.B) {
 	}
 }
 
+func BenchmarkCommonTimeLayouts(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, commonLayout := range []string{"2019-04-29", "2017-05-30T00:00:00Z"} {
+			_, err := StringToDateInDefaultLocation(commonLayout, time.UTC)
+			if err != nil {
+				b.Fatal(err)
+			}
+		}
+	}
+}
+
 func TestIndirectPointers(t *testing.T) {
 	c := qt.New(t)
 

caste.go

@@ -1406,6 +1406,8 @@ func (f timeFormat) hasTimezone() bool {
 
 var (
 	timeFormats = []timeFormat{
+		// Keep common formats at the top.
+		{"2006-01-02", timeFormatNoTimezone},
 		{time.RFC3339, timeFormatNumericTimezone},
 		{"2006-01-02T15:04:05", timeFormatNoTimezone}, // iso8601 without timezone
 		{time.RFC1123Z, timeFormatNumericTimezone},
@@ -1421,7 +1423,6 @@ var (
 		{time.UnixDate, timeFormatNamedTimezone},
 		{time.RubyDate, timeFormatNumericTimezone},
 		{"2006-01-02 15:04:05Z07:00", timeFormatNumericTimezone},
-		{"2006-01-02", timeFormatNoTimezone},
 		{"02 Jan 2006", timeFormatNoTimezone},
 		{"2006-01-02 15:04:05 -07:00", timeFormatNumericTimezone},
 		{"2006-01-02 15:04:05 -0700", timeFormatNumericTimezone},