J2N.Numerics: Extended Span<char> support to net45 and netstandard2.0

[NightOwl888]

New APIs added for netstandard2.0 and net45 targets only

• All Parse()/TryParse() overloads that accept ReadOnlySpan<char> on net6.0

APIs added < net6.0

namespace System
{
    public interface ISpanFormattable
    {
         bool TryFormat(Span<char> destination, out int charsWritten, ReadOnlySpan<char> format, IFormatProvider? provider)
    }
}

New APIs

namespace J2N.Numerics
{
    public sealed class Byte
    {
        public static byte Parse(ReadOnlySpan<char> s, int radix)
        public override bool TryFormat(Span<char> destination, out int charsWritten, ReadOnlySpan<char> format = default, IFormatProvider? provider = null)
        public static bool TryFormat(byte value, Span<char> destination, out int charsWritten, ReadOnlySpan<char> format = default, IFormatProvider? provider = null)
        public static bool TryParse(ReadOnlySpan<char> s, int radix, out byte result)
    }

    public sealed class Double
    {
        public override bool TryFormat(Span<char> destination, out int charsWritten, ReadOnlySpan<char> format = default, IFormatProvider? provider = null)
        public static bool TryFormat(double value, Span<char> destination, out int charsWritten, ReadOnlySpan<char> format = default, IFormatProvider? provider = null)
    }

    public sealed class Int16
    {
        public static short Parse(ReadOnlySpan<char> s, int radix)
        public override bool TryFormat(Span<char> destination, out int charsWritten, ReadOnlySpan<char> format = default, IFormatProvider? provider = null)
        public static bool TryFormat(short value, Span<char> destination, out int charsWritten, ReadOnlySpan<char> format = default, IFormatProvider? provider = null)
        public static bool TryParse(ReadOnlySpan<char> s, int radix, out short result)
    }

    public sealed class Int32
    {
        public static int Parse(ReadOnlySpan<char> s, int radix)
        public override bool TryFormat(Span<char> destination, out int charsWritten, ReadOnlySpan<char> format = default, IFormatProvider? provider = null)
        public static bool TryFormat(int value, Span<char> destination, out int charsWritten, ReadOnlySpan<char> format = default, IFormatProvider? provider = null)
        public static bool TryParse(ReadOnlySpan<char> s, int radix, out int result)
    }

    public sealed class Int64
    {
        public static long Parse(ReadOnlySpan<char> s, int radix)
        public override bool TryFormat(Span<char> destination, out int charsWritten, ReadOnlySpan<char> format = default, IFormatProvider? provider = null)
        public static bool TryFormat(long value, Span<char> destination, out int charsWritten, ReadOnlySpan<char> format = default, IFormatProvider? provider = null)
        public static bool TryParse(ReadOnlySpan<char> s, int radix, out long result)
    }

    public abstract class Number
    {
        public virtual bool TryFormat(Span<char> destination, out int charsWritten, ReadOnlySpan<char> format, IFormatProvider? provider)
    }

    public sealed class SByte
    {
        public static sbyte Parse(ReadOnlySpan<char> s, int radix)
        public override bool TryFormat(Span<char> destination, out int charsWritten, ReadOnlySpan<char> format = default, IFormatProvider? provider = null)
        public static bool TryFormat(sbyte value, Span<char> destination, out int charsWritten, ReadOnlySpan<char> format = default, IFormatProvider? provider = null)
        public static bool TryParse(ReadOnlySpan<char> s, int radix, out sbyte result)
    }

    public sealed class Single
    {
        public override bool TryFormat(Span<char> destination, out int charsWritten, ReadOnlySpan<char> format = default, IFormatProvider? provider = null)
        public static bool TryFormat(float value, Span<char> destination, out int charsWritten, ReadOnlySpan<char> format = default, IFormatProvider? provider = null)
    }
}

New Features

• Support AllowTypeSpecifier on integral types
  • Decimal format (base 10 - without AllowHexSpecifier) allows 'D', 'd', 'F', 'f', 'M', 'm', 'UL', 'Ul', 'uL', 'ul', 'LU', 'Lu', 'lU', or 'lu' after the number
  • Hex format (with AllowHexSpecifier) allows 'UL', 'Ul', 'uL', 'ul', 'LU', 'Lu', 'lU', or 'lu' after the number
• Extended support for AllowTypeSpecifier on decimal types to include 'UL', 'Ul', 'uL', 'ul', 'LU', 'Lu', 'lU', or 'lu' after the number in addition to 'D', 'd', 'F', 'f', 'M', and 'm'
• Adds a dependency on System.Memory for netstandard2.0 and net45 (primarily to allow Span<T> and ReadOnlySpan<T> support)

Optimizations

• ValueStringBuilder ported from .NET 6.0 to replace StringBuilder in number formatters
• Re-added unsafe pointers that were in the original .NET 6.0 formatters

Code Cleanup

• Removes FEATURE_READONLYSPAN and normalizes all of the code to use FEATURE_SPAN