PERFORMANCE: J2N.Text.StringBuilderExtensions: Optimizations

[NightOwl888]

New APIs:

namespace J2N.Text
{
    public static class StringBuilderExtensions
    {
        public static StringBuilder Append(this StringBuilder text, ReadOnlySpan<char> charSequence);
        public static StringBuilder Insert(this StringBuilder text, int index, ReadOnlySpan<char> charSequence);
        public static StringBuilder InsertCodePoint(this StringBuilder text, int index, int codePoint);
    }
}

Optimized APIs:

• Append()
• AppendCodePoint()
• CompareToOrdinal()
• IndexOf()
• Insert()
• LastIndexOf()
• Replace()
• Reverse()

This also fixes several bugs in ValueStringBuilderArrayPoolIndexer and fixes #58.

These optimizations take advantage of ArrayPool, ValueStringBuilderArrayPoolIndexer, and ValueStringBuilderChunkIndexer to provide faster indexing through the StringBuilder class.