Add scopes to temporary variables

[tmedwards]

What it says on the tin.

Have container/non-void macros automatically create a new scope for temporary variables, which is removed after the handler completes.

Likely implementation is a prototype-chain stack.

[hituro]

I recently threw together a version of <<widget>> that created a _local object for temporary variables with a scope to the widget only. That seemed better than shadowing every currently existing temporary variable, which would still allow locals to be polluted by the outside context.

The current behaviours also allows you to use temporary variables as a vehicle to return data from the widget, which is a common use. (Same approach I took with _return in <<function>>)

Might it be a better option to have a another sigil for variables that exist only for the body of the macro? (Hard, I know, due to the lack of additional sigils!)

I can definitely see the use in <<widget>>, I'm not sure this is something I've ever wanted in any other macro, unless it would remove the need to <<capture>> around an async element.