Nested function calls

There is a problem with nested function calls.

Example:
```lua
require "ifj21"

function foo(p : integer) : string
	return "hello"
end

function bar(a : integer, b : integer) : integer
	return a + b
end

function main()
	local test : string = foo(bar(1, 2))
	write(test .. "\n")
end

main()
```

Output:
```
Error at line: 403
Symbol already exists!
```