seen: check for explicit tables on dotted keys (#639)

The TOML spec is being clarified to say that dotted keys "define" their
intermediate tables. Therefore the seen tracker needs to verify that none of
them reference an explicit table.

Also added a missing seen expression check for key-values parsed as part of a
table section.

See https://github.com/toml-lang/toml/issues/846

unmarshaler.go

@@ -516,6 +516,11 @@ func (d *decoder) handleKeyValues(v reflect.Value) (reflect.Value, error) {
 			break
 		}
 
+		err := d.seen.CheckExpression(expr)
+		if err != nil {
+			return reflect.Value{}, err
+		}
+
 		x, err := d.handleKeyValue(expr, v)
 		if err != nil {
 			return reflect.Value{}, err