Factor parse_vector_range out into its own function.

This also fixes the parsing a bit. It was consuming 1 fewer token than
required.

passes/techmap/libparse.h

@@ -106,6 +106,7 @@ namespace Yosys
 		int lexer(std::string &str);
 
     void report_unexpected_token(int tok);
+    int parse_vector_range(int tok);
 		LibertyAst *parse();
 		void error() const;
 		void error(const std::string &str) const;