Move binary and unary operators to external functions

[bvssvni]

Instead of treating binary and unary operators as special, they can be turned into external functions. This will simplify the runtime quite a bit.

[bvssvni]

One problem is that when overriding external functions, the type checker makes wrong assumptions.

[bvssvni]

The runtime type is not guaranteed, so it might be acceptable to just let the type checker make wrong assumptions and get errors at runtime when something is wrong.

[bvssvni]

Another solution is to make a type error if one declares a function with same name and incompatible type as the one assumed by the type checker.

[bvssvni]

Another solution is to add smarter type inference such that the type checker can handle all the cases where the output type depends on the type of the input.

[bvssvni]

One problem is that && and || are lazy, which means they do not always evaluate the right argument.

[bvssvni]

A solution to the lazy problem is to add support for making external functions lazy, e.g. by specifying a default value to return, if the argument is evaluated to that value. This requires lazy functions to return same type as the argument which has a default value.