New CollapseStack plugin

Next steps:

* Catch warnings too (I think this needs core management of warnings, just like what exists for errors)
* Provide a `print_error` (and `print_warning`) to plugins so the output can be colored, etc. I *think* `print_error` should include `mangle_error` (which I can deal with in the CollapseStack plugin)
* @doy also suggested using `Carp::Always` as part of this. +1