Changed required alignment of lookahead_size to 64 bits

This is to prepare for future compatibility with other implementations
of the allocator's lookahead that are under consideration. The most
promising design so far is a sort of segments-list data structure that
stores pointer+size pairs, requiring 64-bits of alignment.

Changing this now takes advantage of the major version to avoid a
compatibility break in the future. If we end up not changing the
allocator or don't need 64-bit alignment we can easily drop this
requirement without breaking anyone's code.

lfs.c

@@ -3095,8 +3095,8 @@ static int lfs_init(lfs_t *lfs, const struct lfs_config *cfg) {
     lfs_cache_zero(lfs, &lfs->rcache);
     lfs_cache_zero(lfs, &lfs->pcache);
 
-    // setup lookahead, must be multiple of 32-bits
-    LFS_ASSERT(lfs->cfg->lookahead_size % 4 == 0);
+    // setup lookahead, must be multiple of 64-bits
+    LFS_ASSERT(lfs->cfg->lookahead_size % 8 == 0);
     LFS_ASSERT(lfs->cfg->lookahead_size > 0);
     if (lfs->cfg->lookahead_buffer) {
         lfs->free.buffer = lfs->cfg->lookahead_buffer;