Collapsed lfsr_ftree_t struct into lfsr_file_t

One less struct to worry about, and less code/stack pressure from
passing around multiple pointers.

There were some naming collisions:

- lfsr_ftree_size -> lfsr_file_bsize
- lfsr_ftree_read -> lfsr_file_read_
- lfsr_ftree_flush -> lfsr_file_flush_

I'm not sure this should be the final result. There are definitely some
rough spots, the hacky "pseudo-file" in lfsr_traversal_t for example.
Having a name specific to file btrees was also useful for naming/
documenting things...

But the code savings are hard to shake a stick at:

            code          stack
  before:  33260           3024
  after:   32874 (-1.2%)   2952 (-2.4%)

lfs.h

@@ -524,12 +524,13 @@ typedef struct lfsr_shrub {
     lfs_size_t estimate;
 } lfsr_shrub_t;
 
-// the lfsr_ftree_t struct is a sort of proto-file
-typedef struct lfsr_ftree {
-    // ftrees contain both an active tree and staging tree, to allow
-    // staging files during mdir compacts
+typedef struct lfsr_file {
+    lfsr_openedmdir_t *next;
+
+    // files contain both an active tree and staging tree, to allow
+    // staging during mdir compacts
     //
-    // navigating this union is a bit tricky, and relies on related
+    // navigating this union is a bit tricky, and relies on the related
     // mdir's block:
     //
     // sign(size)=1, data.size==0           => bnull
@@ -538,19 +539,15 @@ typedef struct lfsr_ftree {
     // sign(size)=0, data.block==mdir.block => bshrub
     // sign(size)=0, data.block!=mdir.block => btree
     //
+    lfsr_mdir_t mdir;
     union {
-        lfs_soff_t size;
+        lfs_soff_t bsize;
         lfsr_data_t bsprout;
         lfsr_bptr_t bptr;
         lfsr_shrub_t bshrub;
         lfsr_btree_t btree;
     } u, u_;
-} lfsr_ftree_t;
 
-typedef struct lfsr_file {
-    lfsr_openedmdir_t *next;
-    lfsr_mdir_t mdir;
-    lfsr_ftree_t ftree;
     uint32_t flags;
     lfs_off_t pos;