Reverted inlined lfsr_data_t representation

See previous commit for more details on why this doesn't work:

1. Losing the simple/compiler friendly lfsr_data_t costs more code/stack
   than we save inlined small pieces of data (dids, leb128s, flags,
   etc).

2. Inlined lfsr_data_t is fundamentally incompatible with the new
   lightweight lfsr_rat_t representation for simple data.

Though I did add a comment, and marked lfsr_data_fromslice as inline.

The compiler was apparently already inlining lfsr_data_fromslice (and
it's a valuable optimization!), but making this explicit helps document/
influence future changes.

Code changes:

           code          stack          ctx
  before: 38128           2672          752
  after:  38060 (-0.2%)   2608 (-2.4%)  752 (+0.0%)

lfs.h

@@ -613,15 +613,16 @@ typedef struct lfsr_omdir {
 //} lfs_mdir_t;
 
 // either an on-disk or in-device data pointer
+//
+// note, it's enticing to make this fancier, but we benefit quite a lot
+// from the compiler being able to aggresively optimize this struct
+//
 typedef struct lfsr_data {
-    // the top bits of size indicate the exact encoding:
-    // top2(size)=0b00 => in-RAM buffer
-    // top2(size)=0b01 => inlined data
-    // top2(size)=0b1x => on-disk reference
+    // sign(size)=0 => in-RAM buffer
+    // sign(size)=1 => on-disk reference
     lfs_size_t size;
     union {
         const uint8_t *buffer;
-        uint8_t imm[8];
         struct {
             lfs_block_t block;
             lfs_size_t off;