refactor: revisit the function declarations

[cristianoliveira]

I am thinking that would be great if the language could express the function declarations in a more math way like:

sum = fn(x, y) { x + y };
return sum(1, 4)

or even:

fn sum(x, y) = { x + y };

return sum(1, 4) 

Just thinking

[cristianoliveira]

First part done in 9529402: let foo = fn [x] { x + 1 }