"match_args" is set in StructMetaInfo and eventually StructMeta via -
```
 info->match_args = PyTuple_GetSlice(info->fields, 0, nfields - nkwonly);
```
This is incref'd before copying to StructMeta, and decref'd when
cleaning up StructMetaInfo. There's no corresponding decref for
StructMeta itself. This causes a leak that valgrind readily detects:
```
env PYTHONMALLOC=malloc valgrind python3 -c 'import msgspec'
...
==576283== LEAK SUMMARY:
==576283==    definitely lost: 1,608 bytes in 26 blocks
```
With the fix in place, we get:
```
==576086== LEAK SUMMARY:
==576086==    definitely lost: 0 bytes in 0 blocks
```