Adopted mask bits for tag lookup/append

This lets us cram in one more mask for potential redund bits:

  name                 tag    mask
  LFSR_TAG_MASK0    0x0000  0x0fff  ---- 1111 1111 1111
  LFSR_TAG_MASK2    0x1000  0x0ffc  ---- 1111 1111 11--
  LFSR_TAG_MASK8    0x2000  0x0f00  ---- 1111 ---- ----
  LFSR_TAG_MASK12   0x3000  0x0000  ---- ---- ---- ----
                                    '.-' '.-' '---.---'
                          mode bits -'    |       |   ^
                            suptype ------'       |   |
                            subtype --------------'   |
                        redund bits ------------------'

I toyed around with a bitwise alternative to the lookup table, but
couldn't come up with anything simpler than these:

- 0xfff & ~((((1<<((i>>1)*8))-1) << ((i&1)*4)) | ((1<<(i*2))-1))
- 0xfff & ~((1 << (((i>>1)*8)+((i&1)<<(1+(i>>1)))))-1)
- 0xfff & ~((1<<(2*i*i))-1) (requires multiply and 32-bit shift)

---

This also replaces the mdir/rbyd/btree/mtree lookup/sublookup/suplookup
functions with a single flexible lookup function that accepts tag masks.

This ended up adding a bit of code/stack (the extra NULL args are
surprisingly pricey), but will hopefully make the redund bits
easier/cheaper to use:

           code          stack          ctx
  before: 35548           2472          636
  after:  35584 (+0.1%)   2480 (+0.3%)  636 (+0.0%)

tests/test_mount.toml

@@ -1132,7 +1132,7 @@ code = '''
             &mdir, NULL, &did) => 0;
     lfsr_mdir_commit(&lfs, &mdir, LFSR_RATTRS(
             LFSR_RATTR_CAT(
-                LFSR_TAG_SUB | (LFSR_TAG_NAME + 0x13), 0,
+                LFSR_TAG_MASK8 | (LFSR_TAG_NAME + 0x13), 0,
                 lfsr_data_fromleb128(did, (uint8_t[LFSR_LEB128_DSIZE]){0}),
                 LFSR_DATA_BUF(path, lfsr_path_namelen(path))))) => 0;
     lfsr_unmount(&lfs) => 0;