Shoehorned in hacky implementation of inline files

Proof-of-concept implementation of inline files that stores the file's
content directly in its parent's directory pair.

Inline files are indicated by a different type stored in an entry's
struct field, and take advantage of resizable entries. Where a normal
file's entry would normally hold the reference to the CTZ skip-list, an
inline file's entry contains the contents of the actual file.

Unfortunately, storing the inline file on disk is the easy part. We also
need to manage inline files in the internals of littlefs and provide the
same operations that we do on normal files, all while reusing as much
code as possible to avoid a significant increase in code cost.

There is a relatively simple, though maybe a bit hacky, solution here. If a
file fits entirely in a cache line, the file logic never actually has to go to
disk. This means we can just give the file a "pretend" block (hopefully
one that would assert if ever written to), and carry out file operations
as normal, as long as we catch the file before it exceeds the cache line
and write out the file to an actual disk.

tests/test_dirs.sh

@@ -357,7 +357,7 @@ tests/test.py << TEST
 TEST
 
 echo "--- Multi-block directory with files ---"
-tests/test.py << TEST
+tests/test.py -s << TEST
     lfs_mount(&lfs, &cfg) => 0;
     lfs_mkdir(&lfs, "prickly-pear") => 0;
     for (int i = 0; i < $LARGESIZE; i++) {