Feature request: allow for not having to redeclare function types

[lucillablessing]

I wish that if a function was forward-declared, as in

local add: function(number, number): number

-- ...

function add(x: number, y: number): number
   return x + y
end

then I didn't need to redeclare the types of the arguments and the result, i.e. I could write this:

local add: function(number, number): number

-- ...

function add(x, y)
   return x + y
end

Obviously this conflicts with functions that have no return value, so if this was ever implemented, some other part of Teal semantics would need to change in a backwards-incompatible way.