disallow extending records with functions outside of original scope

[hishamhm]

tl supports the common pattern of adding functions to an empty table without having to declare them beforehand, which is used in module declarations:

local foo = {}

function foo.fun()
end

A side effect to this was #39, where arbitrary functions could be created by callers of a module, when making a typo while trying to redefine a function (e.g. for a callback, like love.draw).

This restricts the definition of undeclared functions to the same scope level where a record is declared only. So the above example works, but trying to create new functions from other files don't. to declare functions from other files, you have to declare a record type and declare the function as a field to that record, like this:

-- love.d.tl
global love = record
   draw: function()
end

-- main.tl
function love.draw() -- this works
end

function love.bla() -- this doesn't
end

Restricting it by scope might be overly restrictive (a simple wrapper do block will cause a function to be in a different scope and be rejected) and an alternative to consider could be to do it by file, but there's always the alternative of forward-declaring functions first (like tl itself does for tl.process and tl.type_check for example in lines 2 and 3), so this limitation shouldn't be a show-stopper. So I think this behavior should be fine for now.

Fixes #39.