Event listener cannot trigger the same event

[naves-thiago]

Calling the event's __call() method from within a listener would cause __call() to reenter, which is not handled at all (nor prevented) supported in the current implementation.
This is a non-obvious problematic example:

task_t:new(function()
    await("foo")
    -- [...]
    emit("foo")
end)()

[naves-thiago]

The nested_events branch has an implementation to allow nested event calls. This is not well tested.
That implementation has the following semantics:
Any changes to the event from outside one of the listeners take effect immediately (same as it was before).
Any changes (add / remove listeners, change listener type) from inside a listener only affect the subsequent executions (that may be inside the current one).
Changes from a nested execution do not affect the outer execution: If F was a listener when the outer execution started, it will be executed even if a nested execution removed it.

Examples:

t = require"tasks"
e = t.event_t:new()
function f()
    print("f")
    e:listen(function() print("g") end)
end
e:listen(f)
e() -- Prints f
e() -- Prints f and g

t = require"tasks"
e = t.event_t:new()
function a() e:remove_listener(b) print("a", e:listener_count()) end
function b() e:remove_listener(a) print("b", e:listener_count()) end
e:listen(a)
e:listen(b)
e() -- Prints:
-- a	2
-- b	2
e() -- Prints nothing

t = require"tasks"
e = t.event_t:new()
flag = false -- Needed otherwise f() will recurse indefinitely 
function a() print("a") end
function b() print("b") end
function c() print("c") end
function f() if flag then return end flag = true e:listen(b) e() e:listen(c) end
e:listen(a)
e:listen(f)
e() -- Prints: a b a
flag = false
e() -- Prints: c a b c b a

[naves-thiago]

Added an assert to prevent reentering in the master version for now.