Assert on opened/closed file handles

This adopts upstream opened/closed assertions, which are useful for
catching user mistakes (note the bug fixes in our tests):

- Assert if already open in lfsr_*_open
- Assert if not open in lfsr_file_* and lfsr_dir_* functions
- Assert if any files/dirs are still open in lfsr_unmount

Unfortunately this had a surprising code cost for what really should
have been a noop as far as the compiler is concerned. And saved a bit of
stack? Maybe our assertion hints are causing a surprising amount of code
movement? I'm really not sure what's going on and this deserves more
investigation:

           code          stack
  before: 33686           2592
  after:  33904 (+0.6%)   2584 (-0.3%)

At the very least this didn't add a noticable amount of testing time. I
was a bit concerned because our orphan/zombie testing grows opened-list
operations ~O(n^2), but any measurable overhead is less than how much
our test runtime swings between runs (+-~20s).

tests/test_exhaustion.toml

@@ -323,6 +323,7 @@ code = '''
                 lfs_ssize_t d = lfsr_file_write(&lfs, &file, wbuf, SIZE);
                 assert(d == SIZE || d == LFS_ERR_NOSPC);
                 if (d == LFS_ERR_NOSPC) {
+                    lfsr_file_close(&lfs, &file) => 0;
                     goto dead;
                 }
                 err = lfsr_file_close(&lfs, &file);

tests/test_forphans.toml

@@ -581,6 +581,7 @@ code = '''
         assert(memcmp(wbuf, rbuf, CHUNK) == 0);
     }
 
+    lfsr_file_close(&lfs, &file__) => 0;
     lfsr_unmount(&lfs) => 0;
 '''
 
@@ -781,6 +782,7 @@ code = '''
         assert(memcmp(wbuf, rbuf, CHUNK) == 0);
     }
 
+    lfsr_file_close(&lfs, &file) => 0;
     lfsr_unmount(&lfs) => 0;
 '''