use @fieldParentPtr to access your context fields, which lie if you
struct that contains a rb.Tree member (without a pointer).
Also simplifies the init() function so rb.Tree can be initialized in a single line,
without having to use "undefined".
* `std.mem.Compare` is now `std.math.Order` and the enum tags
renamed to follow new style convention.
* `std.mem.compare` is renamed to `std.mem.order`.
* new function `std.math.order`