Misc config loading fixes

The main motivation behind this is simplicity and correctnes, but the new small config library is also faster:

```
BenchmarkDefaultConfigProvider/Viper-16         	  252418	      4546 ns/op	    2720 B/op	      30 allocs/op
BenchmarkDefaultConfigProvider/Custom-16        	  450756	      2651 ns/op	    1008 B/op	       6 allocs/op
```

Fixes #8633
Fixes #8618
Fixes #8630
Updates #8591
Closes #6680
Closes #5192

tpl/encoding/encoding.go

@@ -20,6 +20,7 @@ import (
 	"errors"
 	"html/template"
 
+	"github.com/gohugoio/hugo/common/maps"
 	"github.com/spf13/cast"
 )
 
@@ -71,7 +72,7 @@ func (ns *Namespace) Jsonify(args ...interface{}) (template.HTML, error) {
 	case 2:
 		var opts map[string]string
 
-		opts, err = cast.ToStringMapStringE(args[0])
+		opts, err = maps.ToStringMapStringE(args[0])
 		if err != nil {
 			break
 		}