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

• Implement in System.Private.CoreLib in coreclr
• Implement in System.Private.CoreLib in corert (unless string is "shared" by then)
• Expose from System.Runtime contract in corefx
• Add tests to System.Runtime tests in corefx

namespace System
{
    public class String
    {
        public String(ReadOnlySpan<char> value);
        …
    }
}