bitfield slots: an idea that didn't work out

[jepler]

I was bothered by the presumed waste of using 12 bytes to track which type slots are filled, rather than 12 bits.

So today I did the work to adapt the basic object structure like so: (full code at jepler@bitfield-slots)

// This struct is a variable sized struct. In order to use this as a         
// member, or allocate dynamically, use the mp_obj_empty_type_t or
// mp_obj_full_type_t structs below (which must be kept in sync).   
struct _mp_obj_type_t {
    // A type is an object so must start with this entry, which points to mp_type_type.
    mp_obj_base_t base;

    // Flags associated with this type.
    uint16_t flags;
    
    // The name of this type, a qstr.
    uint16_t name;
    
    // Slots: If a slot is populated, the corresponding bit in `filled_slots` is set.
    // The index in `slots[]` is simply the number of set bits "below" that one,
    // which can be determined with __builtin_popcnt().
    uint16_t filled_slots;
    // (16 spare bits here on most ports!)

    const void *slots[];
};

.. and changed the rest of the core as needed until unix make VARIANT=coverage test passed.

Then I noticed that cortex m4 (and everything below it) lacks a popcnt instruction. So this saved a mere 4 bytes on the "itsybitsy" build, and probably tanked performance due to all the introduced __popcountsi2 calls.. And it makes the MP_DEFINE_CONST_OBJ_TYPE_NARGS_ macros much more gruesome.