Changed callbacks to take user-provided context directly

This is a style change to make littlefs's callbacks consistent with most
callback declarations found in C. That is, taking in a user-provided
`void*`.

Previously, these callbacks took a pointer to the config struct itself,
which indirectly contained a user provided context, and this gets the
job done, but taking in a callback with a `void*` is arguably more
expected, has a better chance of integrating with C++/OS-specific code,
and is more likely to be optimized out by a clever compiler.

---

As a part of these changes, the geometry for the test bds needed to be
moved into bd specific configuration objects. This is a good change as
it also allows for testing situations where littlefs's geometry does not
match the underlying bd.

tests/test_evil.toml

@@ -158,12 +158,12 @@ code = '''
     lfs_ctz_fromle32(&ctz);
     // rewrite block to contain bad pointer
     uint8_t bbuffer[LFS_BLOCK_SIZE];
-    cfg.read(&cfg, ctz.head, 0, bbuffer, LFS_BLOCK_SIZE) => 0;
+    lfs_testbd_read(&bd, ctz.head, 0, bbuffer, LFS_BLOCK_SIZE) => 0;
     uint32_t bad = lfs_tole32(0xcccccccc);
     memcpy(&bbuffer[0], &bad, sizeof(bad));
     memcpy(&bbuffer[4], &bad, sizeof(bad));
-    cfg.erase(&cfg, ctz.head) => 0;
-    cfg.prog(&cfg, ctz.head, 0, bbuffer, LFS_BLOCK_SIZE) => 0;
+    lfs_testbd_erase(&bd, ctz.head) => 0;
+    lfs_testbd_prog(&bd, ctz.head, 0, bbuffer, LFS_BLOCK_SIZE) => 0;
     lfs_deinit(&lfs) => 0;
 
     // test that accessing our bad file fails, note there's a number