Add a very simple test for Parser::to_linecol.

This test shows that in some cases converting error offsets generated by
the parser to line and column numbers will panic ('arithmetic operation
overflowed') in debug mode, and return an incorrect column number in
release mode.

tests/invalid.rs

@@ -7,6 +7,12 @@ fn run(toml: &str) {
     let table = p.parse();
     assert!(table.is_none());
     assert!(p.errors.len() > 0);
+
+    // test Parser::to_linecol with the generated error offsets
+    for error in &p.errors {
+      p.to_linecol(error.lo);
+      p.to_linecol(error.hi);
+    }
 }
 
 macro_rules! test( ($name:ident, $toml:expr) => (