Handler closure capture

Lambdas defined inside an on handler capture its local variables by value, including callbacks passed to map and all and nested callbacks. Those locals remain available if the closure escapes the handler. Global variables continue to be read live; capturing handler locals does not freeze globals.

Each handler invocation has its own local frame. A closure retains the locals of its defining invocation, rather than reading another invocation's locals. Assignment inside a closure continues to bind a function-local variable.