Before this commit, there were 2 different ways of generating variable
identifiers. And it was possible to have a situation with two different
variable in a template with the same identifier, which caused a crash.
This commit ensures that we go through a unique helper method, so this
cannot occur anymore. Also, the code is slightly simpler.
We add some test for the t-on directive.
For making them pass, it was necessary to change the code produced by
compileTForeach: the const declaration is not done by using generateId
and there was some conflict with the variable names produced in
captureExpression. Consequently, many snapshots had to be changed.
Code prettification has been done too.