trv: Split lfs3_trv_t -> lfs3_trv_t, lfs3_mgc_t, and lfs3_mtrv_t

A big downside of LFS3_T_REBUILDGBMAP is the addition of an lfs3_btree_t
struct to _every_ traversal object.

Unfortunately, I don't see a way around this. We need to track the new
gbmap snapshot _somewhere_, and other options (such as a global gbmap.b_
snapshot) just move the RAM around without actually saving anything.

To at least mitigate this internally, this splits lfs3_trv_t into
distinct lfs3_trv_t, lfs3_mgc_t, and lfs3_mtrv_t structs that capture
only the relevant state for internal traversal layers:

- lfs3_mtree_traverse <- lfs3_mtrv_t
- lfs3_mtree_gc       <- lfs3_mgc_t (contains lfs3_mtrv_t)
- lfs3_trv_read       <- lfs3_trv_t (contains lfs3_mgc_t)

This minimizes the impact of the gbmap rebuild snapshots, and saves a
big chunk of RAM. As a plus it also saves RAM in the default build by
limiting the 2-block block queue to the high-level lfs3_trv_read API:

                 code          stack          ctx
  before:       37176           2360          684
  after:        37176 (+0.0%)   2352 (-0.3%)  684 (+0.0%)

                 code          stack          ctx
  gbmap before: 40060           2432          848
  gbmap after:  40024 (-0.1%)   2368 (-2.6%)  848 (+0.0%)

The main downside? Our field names are continuing in their
ridiculousness:

  lfs3.gc.gc.t.b.h.flags // where else would the global gc flags be?

tests/test_alloc.toml

@@ -207,8 +207,8 @@ code = '''
     uint8_t *seen = malloc((BLOCK_COUNT+7)/8);
     memset(seen, 0, (BLOCK_COUNT+7)/8);
 
-    lfs3_trv_t trv;
-    lfs3_trv_init(&trv,
+    lfs3_mtrv_t mtrv;
+    lfs3_mtrv_init(&mtrv,
             LFS3_T_RDONLY
                 | ((CKMETA) ? LFS3_T_CKMETA : 0));
     for (lfs3_block_t i = 0;; i++) {
@@ -217,7 +217,7 @@ code = '''
 
         lfs3_stag_t tag;
         lfs3_bptr_t bptr;
-        tag = lfs3_mtree_traverse(&lfs3, &trv,
+        tag = lfs3_mtree_traverse(&lfs3, &mtrv,
                 &bptr);
         assert(tag >= 0 || tag == LFS3_ERR_NOENT);
         if (tag == LFS3_ERR_NOENT) {
@@ -381,8 +381,8 @@ code = '''
     uint8_t *seen = malloc((BLOCK_COUNT+7)/8);
     memset(seen, 0, (BLOCK_COUNT+7)/8);
 
-    lfs3_trv_t trv;
-    lfs3_trv_init(&trv,
+    lfs3_mtrv_t mtrv;
+    lfs3_mtrv_init(&mtrv,
             LFS3_T_RDONLY
                 | ((CKMETA) ? LFS3_T_CKMETA : 0));
     for (lfs3_block_t i = 0;; i++) {
@@ -391,7 +391,7 @@ code = '''
 
         lfs3_stag_t tag;
         lfs3_bptr_t bptr;
-        tag = lfs3_mtree_traverse(&lfs3, &trv,
+        tag = lfs3_mtree_traverse(&lfs3, &mtrv,
                 &bptr);
         assert(tag >= 0 || tag == LFS3_ERR_NOENT);
         if (tag == LFS3_ERR_NOENT) {
@@ -541,8 +541,8 @@ code = '''
     uint8_t *seen = malloc((BLOCK_COUNT+7)/8);
     memset(seen, 0, (BLOCK_COUNT+7)/8);
 
-    lfs3_trv_t trv;
-    lfs3_trv_init(&trv,
+    lfs3_mtrv_t mtrv;
+    lfs3_mtrv_init(&mtrv,
             LFS3_T_RDONLY
                 | ((CKMETA) ? LFS3_T_CKMETA : 0));
     for (lfs3_block_t i = 0;; i++) {
@@ -551,7 +551,7 @@ code = '''
 
         lfs3_stag_t tag;
         lfs3_bptr_t bptr;
-        tag = lfs3_mtree_traverse(&lfs3, &trv,
+        tag = lfs3_mtree_traverse(&lfs3, &mtrv,
                 &bptr);
         assert(tag >= 0 || tag == LFS3_ERR_NOENT);
         if (tag == LFS3_ERR_NOENT) {