Add support for Python 3.8 fstring debug = (microsoft#1548)

* Add equals specifier, less error testing

* Whitespace -> WhiteSpace for consistency

* Add error reporting

* Cleanups

* PR feedback

* Use var more consistently

Author: jakebailey

src/Core/Impl/Text/StringSpan.cs

@@ -30,6 +30,8 @@ public struct StringSpan : IEquatable<StringSpan> {
         public int End => Start + Length;
         public bool IsValid => Source != null && Start >= 0 && Length >= 0 && Source.Length >= End;
 
+        public StringSpan(string source) : this(source, 0, source.Length) { }
+
         public StringSpan(string source, int start, int length) {
             Source = source;
             Start = start;