rule: check for switch over *Enum or *TaggedUnion

[DonIsaac]

Consider the following code:

const Foo = union(enum) {
  a: i32,
  b: []const u8,

  fn deinit(self: *Foo, alloc: Allocator) void {
    // WRONG! switching over pointer, not enum
    // should be `switch(self.*)`
    switch(self) {
      .a => {}, // noop
      .b => alloc.free(self.b),
    }
  }
};

The zig compiler provides this error message:

example.zig:8:5: error: else prong required when switching on type '*src.example.Foo'
        switch (self) {

This message isn't great. Ideally it would have a note saying "did you forget to dereference this pointer?"

We should add a rule that checks for this.