Feature request: support usage before declaration for method

[BLKTower]

Giving:

local record mod
end

function mod:foo1()
    self:foo2() -- error: invalid key 'foo2'
end

function mod:foo2()
end

It'd be nice that teal can support usage before declaration for method.
Desired behavior: No error

[JR-Mitchell]

I just thought I'd note that the following (explicitly declaring the method type before assigning the method itself) does work:

local record mod
    foo2: function(self)
end

function mod:foo1()
    self:foo2() -- no longer errors, as foo2 has been forward declared
end

function mod:foo2()
end