trv: Reduced LFS3_t_CKPOINTED + LFS3_t_MUTATED -> LFS3_t_CKPOINTED

This drops LFS3_t_MUTATED in favor of just using LFS3_t_CKPOINTED
everywhere:

1. These meant roughly the same thing, with LFS3_t_MUTATED being a bit
   tighter at the cost of needing to be explicitly set.

2. The implicit setting of LFS3_t_CKPOINTED by lfs3_alloc_ckpoint -- a
   function that already needs to be called before mutation -- means we
   have one less thing to worry about.

   Implicit properties like LFS3_t_CKPOINTED are great for building a
   reliable system. Manual flags like LFS3_t_MUTATED, not so much.

3. Why use two flags when we can get away with one?

The only downside is we may unnecessarily clobber gc/traversal work when
we don't actually mutate the filesystem. Failed file open calls are a
good example.

However this tradeoff seems well worth it for an overall simpler +
more reliable system.

---

Saves a bit of code:

                 code          stack          ctx
  before:       37220           2352          688
  after:        37160 (-0.2%)   2352 (+0.0%)  688 (+0.0%)

                 code          stack          ctx
  gbmap before: 40184           2368          856
  gbmap after:  40132 (-0.1%)   2368 (+0.0%)  856 (+0.0%)

lfs3.h

@@ -344,10 +344,9 @@ enum lfs3_btype {
 #define LFS3_t_TSTATE   0x000f0000  // The current traversal state
 #define LFS3_t_BTYPE    0x00f00000  // The current block type
 #define LFS3_t_ZOMBIE   0x08000000  // File has been removed
-#define LFS3_t_DIRTY    0x04000000  // Filesystem modified outside traversal
-#define LFS3_t_MUTATED  0x02000000  // Filesystem modified during traversal
+#define LFS3_t_DIRTY    0x04000000  // Filesystem ckpointed outside traversal
 #define LFS3_t_CKPOINTED \
-                        0x01000000  // Filesystem ckpointed during traversal
+                        0x02000000  // Filesystem ckpointed during traversal
 
 // GC flags
 #ifndef LFS3_RDONLY