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_alloc.toml

@@ -362,7 +362,7 @@ code = '''
 
     // but mark the head of our file as a "bad block", this is force our
     // scan to bail early
-    lfs_testbd_setwear(&cfg, fileblock, 0xffffffff) => 0;
+    lfs_testbd_setwear(&bd, fileblock, 0xffffffff) => 0;
     lfs_file_open(&lfs, &file, "ghost", LFS_O_WRONLY | LFS_O_CREAT) => 0;
     strcpy((char*)buffer, "chomp");
     size = strlen("chomp");
@@ -377,7 +377,7 @@ code = '''
 
     // now reverse the "bad block" and try to write the file again until we
     // run out of space
-    lfs_testbd_setwear(&cfg, fileblock, 0) => 0;
+    lfs_testbd_setwear(&bd, fileblock, 0) => 0;
     lfs_file_open(&lfs, &file, "ghost", LFS_O_WRONLY | LFS_O_CREAT) => 0;
     strcpy((char*)buffer, "chomp");
     size = strlen("chomp");