Adopted more single-char field names

Limited to nested struct fields where the names don't really matter:

- bptr.data -> bptr.d
- mdir.rbyd -> mdir.r

Ok it actually just ended up those two.

This is on the tail end of some optimization work that ended up
abandoned because of maintainability concerns. But it did highlight that
struct nesting gets a bit out-of-control when trying to both optimize
stack allocations and respect C99's strict aliasing.

Consider further fragmenting lfs3_rbyd_t for fine-grain stack
allocations:

  typedef struct lfs3_rbyd {
      struct lfs3_rtrunkcksum {
          struct lfs3_rtrunk {
              lfs3_rid_t weight;
              struct lfs3_rtrunktrunk {
                  lfs3_block_t blocks[2];
                  lfs3_size_t trunk;
              } rtrunktrunk;
          } rtrunk;
          uint32_t cksum;
      } rtrunkcksum;
      lfs3_size_t eoff;
  } lfs3_rbyd_t;

Accessing fields just starts to get silly:

  rbyd.rtrunkcksum.rtrunk.trunktrunk.trunk

At least single-char field names keeps a little bit of readability:

  rbyd.ck.t.t.trunk

Or for some real examples:

- file->b.o.mdir.rbyd.weight -> file->b.o.mdir.r.weight
- bptr->data.u.disk.block -> bptr->d.u.disk.block

tests/test_mount.toml

@@ -203,7 +203,7 @@ code = '''
 
     // hack, don't use the internals like this
     uint8_t wbuf[SIZE];
-    while ((file.b.o.mdir.rbyd.eoff & 0x7fffffff) <= GC_COMPACT_THRESH) {
+    while ((file.b.o.mdir.r.eoff & 0x7fffffff) <= GC_COMPACT_THRESH) {
         lfs3_file_rewind(&lfs3, &file) => 0;
         for (lfs3_size_t j = 0; j < SIZE; j++) {
             wbuf[j] = 'a' + (TEST_PRNG(&prng) % 26);
@@ -245,7 +245,7 @@ code = '''
 
     // mdir should have been compacted
     lfs3_file_open(&lfs3, &file, "jellyfish", LFS3_O_RDONLY) => 0;
-    assert((file.b.o.mdir.rbyd.eoff & 0x7fffffff) <= GC_COMPACT_THRESH);
+    assert((file.b.o.mdir.r.eoff & 0x7fffffff) <= GC_COMPACT_THRESH);
 
     // check we can still read the file
     uint8_t rbuf[SIZE];