remove exists function

[lonegunmanb]

An exists function lure users into thinking they can use a not exists() check, or a not_exists() function, which is not correct. When we pass a non-exist path as function's parameter, the whole rule check stops with result as undefined. None of not exists(non_exist_path) nor not_exists(non_exist_path)` works as expected.

Another classic approach is to check existence is like:

rule1 := true if {
    input.foo.bar
}

If we'd like to assert that the path is not existed, we might use:

rule1 := true if {
    not input.foo.bar
}

But this rule would return a false result if input.foo.bar's type is boolean and the value is false.

The correct way to check non-existence is:

rule1 := true if {
    not input.foo.bar == input.foo.bar
}

In rego, == function wouldn't be effected by a non-exist path. For symmetry reasons, we stipulate that in this library, we must check the existence by:

exist := true if {
    input.foo.bar == input.foo.bar
}
not_exist := true if {
    not input.foo.bar == input.foo.bar
}