Built-in function names should be excluded from item names

[kosiakk]

It is possible to define a functor, which shares the name with built-in function. But the built-in function always takes precedence:

:-backchain whatever/1 .
:-backchain abs/1 .
whatever(X) = "hello".
abs(X)      = "hello".  // I'd expect a compiler error here
test1 = whatever(-4).
test2 = abs(-4) .

Changes
=======
test1 = "hello".
test2 = 4.

Since the list of built-in function differs between backends, I'd make it a compiler error to define an item which clashes with built-in function. Regardless of arity, to be on the safe side.