Fix issues with identifier spans

Upsilon/Parser/Lexer.cs

@@ -250,7 +250,7 @@ namespace Upsilon.Parser
             var str = stringBuilder.ToString();
             if (kind == SyntaxKind.Identifier)
             {
-                return new IdentifierToken(str, new TextSpan(startLine, start, _linePosition, _position));
+                return new IdentifierToken(str, new TextSpan(startLine, start, _linePosition, _position + 1));
             }
 
             if (kind == SyntaxKind.ReturnKeyword)

Upsilon/Text/SourceText.cs

@@ -17,6 +17,7 @@ namespace Upsilon.Text
             if (startPosition < 0) startPosition = 0;
             if (startLine == endLine)
             {
+                if (endPosition > _text[startLine].Length) endPosition = _text[startLine].Length;
                 return _text[startLine].Substring(startPosition, endPosition - startPosition);
             }
             var sb = new StringBuilder();

Upsilon/Text/SourceTextLine.cs

@@ -1,15 +0,0 @@
-namespace Upsilon.Text
-{
-    public struct SourceTextLine
-    {
-        public SourceTextLine(int start, int lineLength)
-        {
-            Start = start;
-            LineLength = lineLength;
-        }
-
-        public int Start { get; }
-        public int LineLength { get; }
-        public int End => Start + LineLength;
-    }
-}