String.format is widely used, and improving its performance is very meaningful. This PR can significantly improve the performance of String.format. Sorry, there are many changes, which will take a lot of time. I hope you can review it patiently.

Improved performance includes the following:

1. Write directly during the parse process to reduce object allocation.
In the current Formatter implementation, some objects do not need to be allocated, such as:

class Formatter {
    public Formatter format(Locale l, String format, Object ... args) {
     List<FormatString> fsa = parse(format);
     // ...
    }

    static List<FormatString> parse(String s) {
     ArrayList<FormatString> al = new ArrayList<>();

        while (i < max) {
            int n = s.indexOf('%', i);
            if (n < 0) {
                //
                al.add(new FixedString(s, i, max));
            }
        }
    }
}
In the process of parsing, the content that is not a Specifier is directly appended without going through FixedString. By directly printing the parsed FormatString object, there is no need to construct a List<FormatString> fsa to store it.

2. Fast path print
Use specialized FormatString implementations for single-character and single-width specifiers to avoid calling the large FormatSpecifier#print method.

3. String.format directly calls j.u.Formatter
String.format directly calls j.u.Formatter via SharedSecrets to improve performance