Negating complex16 should either be banned or implementing

[bradunov]

If one writes -c(), where function c() returns complex16, this passes all the way to code generation and gets caught by C compiler with
error C2675: unary '-' : 'complex16' does not define this operator or a conversion to a type acceptable to the predefined operator
We should either throw an exception in the compiler or implement it properly.
(Maybe implementing it in the new codegen is a better idea, since it should be simple and adds another feature)