Add StringBuilder Span-based APIs

[stephentoub]

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

• Implement in System.Private.CoreLib in coreclr ("shared" with corert)
• Expose from System.Runtime contract in corefx
• Add tests to System.Runtime tests in corefx

namespace System.Text
{
    public class StringBuilder
    {
        public StringBuilder Append(ReadOnlySpan<char> value);
        public StringBuilder Insert(int index, ReadOnlySpan<char> value);
        public void CopyTo(int sourceIndex, Span<char> destination, int count);
        …
    }
}

[kevingosse]

Can we start working on these?

[stephentoub]

Yup.

[kevingosse]

Alright, on it.

[stephentoub]

Fixed by dotnet/coreclr#13163 and dotnet/corefx#22852. Thanks, @kookiz.