Tweaked mid to use sign-bit to indicate mroots

This is an intermediate commit as a part of a tangent into compressed
mids.

The idea here, is instead of using bid=-1 as a special value for mroots,
use only the top bit to indicate mroots. This allows you to compare
against the grm/other uninlined mids by masking instead of signed
comparison.

This is valuable for compressed mids since extracting bids relies on
knowledge of the block size, and becomes quite a bit more expensive.

            mroot bid                             mroot cmp
  before:  0xffffffff  lfs_smax32(a, 0) == lfs_smax32(b, 0)
  after:   0x80000000  (a & 0x7fffffff) == (b & 0x7fffffff)

The implementation here is a bit clumsy. I think GCC may be not that
great at optimizing out copies of structs being passed around via
inlined functions. But this is only a proof-of-concept.

tests/test_alloc.toml

@@ -109,7 +109,7 @@ code = '''
             LFSR_ATTR(0, RM, -1, NULL))) => 0;
 
     lfsr_mdir_t mdir;
-    lfsr_mtree_lookup(&lfs, LFSR_MID(lfsr_mtree_weight(&lfs)-1, -1),
+    lfsr_mtree_lookup(&lfs, LFSR_MID(0, -1),
             &mdir) => 0;
     mdir.mid.rid = 0;
 
@@ -144,8 +144,8 @@ code = '''
 
     // test that all of our metadata entries are still there
     lfs_size_t i = 0;
-    for (lfs_ssize_t mid = (lfsr_mtree_isinlined(&lfs) ? -1 : 0);
-            mid < (lfs_ssize_t)lfsr_mtree_weight(&lfs);
+    for (lfs_ssize_t mid = 0;
+            mid < lfs_smax32(lfsr_mtree_weight(&lfs), 1);
             mid++) {
         lfsr_mdir_t mdir;
         lfsr_mtree_lookup(&lfs, LFSR_MID(mid, -1), &mdir) => 0;