WIP: support finalizers in trimming

[JeffBezanson]

This is sufficient to get finalizers working. Before merging this though I propose generalizing it by adding a new builtin calledge (or something) that we'd use like so:

function finalizer(@nospecialize(f), @nospecialize(o))
    _check_mutable(o)
    Core.finalizer(f, o)
    calledge(Tuple{typeof(f), typeof(o)})
    return o
end

function Core.Task(@nospecialize(f), reserved_stack::Int=0)
    calledge(Tuple{typeof(f)})
    Core._Task(f, reserved_stack, ThreadSynchronizer())
end

which says that if a call to finalizer is reachable, then so is f(o) etc. The implementation would be basically identical to what's here currently.

[topolarity]

This needs re-work. We need to at least check that we're not re-inserting an mi to the workqueue or else we can infinite loop because we'll get a fresh ci and continually re-insert a recursive finalizer

Jameson suggested a re-factor of the workqueue to accommodate this, which I've started on but it's not done just yet

[topolarity]

adding a new builtin calledge (or something) that we'd use like so

I was thinking perhaps Core._predeclare_call(foo, a, b)