Add DateTime{Offset} Span-based APIs

[stephentoub]

Separated out of https://github.com/dotnet/corefx/issues/21281 for tracking purposes.

• Implement in System.Private.CoreLib in coreclr (corert shares the same files)
• Expose from System.Runtime contract in corefx
• Add tests to System.Runtime tests in corefx

namespace System
{
    public struct DateTime
    {
        public static DateTime Parse(ReadOnlySpan<char> s, IFormatProvider provider = null, System.Globalization.DateTimeStyles styles = DateTimeStyles.None);
        public static DateTime ParseExact(ReadOnlySpan<char> s, string format, IFormatProvider provider, DateTimeStyles style = DateTimeStyles.None);
        public static DateTime ParseExact(ReadOnlySpan<char> s, string[] formats, IFormatProvider provider, DateTimeStyles style);

        public static bool TryParse(ReadOnlySpan<char> s, out DateTime result, IFormatProvider provider = null, DateTimeStyles styles = DateTimeStyles.None);
        public static bool TryParseExact(ReadOnlySpan<char> s, string format, IFormatProvider provider, DateTimeStyles style, out DateTime result);
        public static bool TryParseExact(ReadOnlySpan<char> s, string[] formats, IFormatProvider provider, DateTimeStyles style, out DateTime result);

        public bool TryFormat(Span<char> destination, out int charsWritten, string format = null, IFormatProvider provider = null);
        …
    }

    public struct DateTimeOffset
    {
        public static DateTimeOffset Parse(ReadOnlySpan<char> input, IFormatProvider formatProvider = null, System.Globalization.DateTimeStyles styles = DateTimeStyles.None);
        public static DateTimeOffset ParseExact(ReadOnlySpan<char> input, string format, IFormatProvider formatProvider, DateTimeStyles styles = DateTimeStyles.None);
        public static DateTimeOffset ParseExact(ReadOnlySpan<char> input, string[] formats, IFormatProvider formatProvider, DateTimeStyles styles);

        public static bool TryParse(ReadOnlySpan<char> input, out DateTimeOffset result, IFormatProvider formatProvider = null, DateTimeStyles styles = DateTimeStyles.None);
        public static bool TryParseExact(ReadOnlySpan<char> input, string format, IFormatProvider formatProvider, DateTimeStyles styles, out DateTimeOffset result);
        public static bool TryParseExact(ReadOnlySpan<char> input, string[] formats, IFormatProvider formatProvider, DateTimeStyles styles, out DateTimeOffset result);

        public bool TryFormat(Span<char> destination, out int charsWritten, string format = null, IFormatProvider formatProvider = null);
        …
    }
}