func (sa *StringArena) NewString(b []byte) string {
	if len(sa.buf)+len(b) > cap(sa.buf) {
		return string(b)
	}
	start := len(sa.buf)
	sa.buf = append(sa.buf, b...)
	return sa.str[start : start+len(b)]
}
NewString copies a byte slice into the arena and returns it as a string.
If the arena is full, it returns a traditional go string.
package hack
import (
	"reflect"
	"unsafe"
)
type StringArena struct {
	buf []byte
	str string
}
func NewStringArena(size int) *StringArena {
	sa := &StringArena{buf: make([]byte, 0, size)}
	pbytes := (*reflect.SliceHeader)(unsafe.Pointer(&sa.buf))
	pstring := (*reflect.StringHeader)(unsafe.Pointer(&sa.str))
	pstring.Data = pbytes.Data
	pstring.Len = pbytes.Cap
	return sa
}