Abandoned data-backed cache, use indirect lfs3_data_t on stack

This abandons the data-backed cache idea due to concerns around
readability and maintainability. Mixing const/mutable buffers in
lfs3_data_t was not great.

Instead, we now just allocate an indirect lfs3_data_t on the stack in
lfs3_file_sync_ to avoid the previous undefined behavior.

This actually results in less stack usage total, due to lfs3_file_t
allocations in lfs3_set/read, and avoid the more long-term memory cost
in lfs3_file_t:

              code          stack          ctx
  before:    36832           2376          684
  after:     36840 (+0.0%)   2368 (-0.3%)  684 (+0.0%)

Oh. And lfs3_file_sync_ isn't even on the stack hot-path, so this is a
net benefit over the previous cache -> data cast:

              code          stack          ctx
  before sa: 36844           2368          684
  after sa:  36840 (-0.0%)   2368 (+0.0%)  684 (+0.0%)

Still less cool though.

lfs3.h

@@ -636,7 +636,6 @@ typedef struct lfs3_data {
     lfs3_size_t size;
     union {
         const uint8_t *buffer;
-        uint8_t *buffer_;
         struct {
             lfs3_block_t block;
             lfs3_size_t off;
@@ -742,11 +741,14 @@ typedef struct lfs3_file {
     #endif
 
     // in-RAM cache
+    //
+    // note this lines up with lfs3_data_t's buffer representation
     struct {
         #ifndef LFS3_KVONLY
         lfs3_off_t pos;
         #endif
-        lfs3_data_t d;
+        lfs3_off_t size;
+        uint8_t *buffer;
     } cache;
 
     // on-disk leaf bptr