Add tracking of what closures capture and syntax to state what to capture. #9
Description
A current problem I have in a problem is lots and lots of upvalues being captured unexpectedly in closures and causing various memory leaks, so it would be nice to have a way to explicitly mark which, if any variables in the enclosing scope(s) should be captured and then emit an error/warning for those that are referenced in the function, but not explicitly marked as captured.
Good inspiration here is C++ lambda syntax, one of few languages (outside of maybe rust too?) with explicit captures.