Add Steve Johnson-style parser recovery rules:

SourceUnit = Error $
    Block = '{' Error '}'
    ContractDefinition = '{' Error '}'
    Statement = Error ';'

Co-Authored-By: chriseth <chris@ethereum.org>

liblangutil/Scanner.h

@@ -110,6 +110,9 @@ public:
 	/// @returns the next token and advances input
 	Token next();
 
+	/// Set scanner to a specific offset. This is used in error recovery.
+	void setPosition(size_t _offset);
+
 	///@{
 	///@name Information about the current token