Replaced attr-list arenas with three independent arrays

lfsr_attr_t attrs[a*d*b];  =>  lfsr_attr_t attrs[a];
  lfs_size_t attr_count;         lfs_size_t attr_count;
  lfs_size_t attr_scratch;       lfsr_data_t datas[d];
                                 lfs_size_t data_count;
                                 uint8_t buf[b];
                                 lfs_size_t buf_size;

This mostly reverts the allocator scaffolding needed for the attr-list
arenas (LFS_ALIGNOF, etc). This is the main draw of this change, as it
would be nice to avoid a low-level arena implementation headaches unless
they prove to be worthwhile. Which they haven't really so far...

Unfortunately this comes with another code cost, I think due to the
number of counters needed to keep track of separate attr/data/buf
allocations. At least stack showed a slight improvement:

           code          stack
  before: 33844           2824
  after:  33872 (+0.1%)   2816 (-0.3%)

lfs_util.h

@@ -221,16 +221,6 @@ static inline void lfs_sswap32(int32_t *a, int32_t *b) {
     *b = t;
 }
 
-// Find alignment of a type at compile time
-#if !defined(LFS_NO_INTRINSICS)
-#define LFS_ALIGNOF(t) __alignof__(t)
-#else
-#define LFS_ALIGNOF(t) ((size_t)&((struct {char a; t b;}*)0)->b)
-#endif
-
-// Find size necessary to align type at compile time
-#define LFS_ALIGNEDSIZEOF(t) (sizeof(t) + LFS_ALIGNOF(t)-1)
-
 // Align to nearest multiple of a size
 static inline uint32_t lfs_aligndown(uint32_t a, uint32_t alignment) {
     return a - (a % alignment);