Boolean pointer validator

The Bool pointer validator provides functions for setting validation rules for a bool pointer, or a custom type based on a bool pointer.

Below is a valid example for every Bool pointer validator rule.

x := true;   v.Is(v.BoolP(&x).EqualTo(true))
x := true;   v.Is(v.BoolP(&x).True())
x := false;  v.Is(v.BoolP(&x).False())
x := true;   v.Is(v.BoolP(&x).Passing(func(val *bool) bool { return val == true }))
x := true;   v.Is(v.BoolP(&x).InSlice([]string{true, false}))
x := false;  v.Is(v.BoolP(&x).FalseOrNil())
var x *bool; v.Is(v.BoolP(x).Nil())