Reverted lfsr_data_t lazily encoded leb128s

- It didn't save code.

- An inlined buffer is potentially more useful, even if only marginally,
  and, uh, unproven yet.

- Requiring lfs_toleb128 in a readonly implementation is a hard ask.

lfs.h

@@ -399,11 +399,17 @@ typedef struct lfs_mdir {
 
 // Either an on-disk or in-device data pointer
 //
-// The top 2 bits of data's size indicates the actual encoding
-// 0b00 => in-RAM buffer
-// 0b01 => a single leb128
-// 0b10 => on-disk reference
-// 0b11 => concatenated datas
+// The sign-bit of the size field indicates if the data is
+// in-device or on-disk.
+//
+// After removing the sign bit, the size always encodes the
+// resulting size on-disk.
+//
+// The exact representation of in-device data also depends on the
+// mode field:
+// - pointer to a RAM-backed buffer
+// - inlined data able to fit at least 1 leb128
+// - an array of concatenated datas
 //
 // Note concatenated datas can only be 1 level deep. Concatenating
 // concatenated datas would require recursion to resolve.
@@ -422,8 +428,8 @@ typedef struct lfsr_data {
         } buf;
         struct {
             lfs_ssize_t size;
-            uint32_t word;
-        } word;
+            uint8_t buf[8];
+        } imm;
         struct {
             lfs_ssize_t size;
             const struct lfsr_data *datas;