CLN: unify fused type definitions
```
grep -r fused pandas/_libs/ --include '*.pyx'
pandas/_libs/writers.pyx:ctypedef fused pandas_string:
pandas/_libs/reshape.pyx:ctypedef fused reshape_t:
pandas/_libs/join.pyx:ctypedef fused join_t:
pandas/_libs/join.pyx:ctypedef fused asof_t:
pandas/_libs/join.pyx:ctypedef fused by_t:
pandas/_libs/algos.pyx:ctypedef fused algos_t:
pandas/_libs/algos.pyx:ctypedef fused rank_t:
pandas/_libs/algos.pyx:ctypedef fused diff_t:
pandas/_libs/algos.pyx:ctypedef fused out_t:
pandas/_libs/groupby.pyx:ctypedef fused add_t:
pandas/_libs/groupby.pyx:ctypedef fused rank_t:
pandas/_libs/groupby.pyx:ctypedef fused groupby_t:
```

cc @mzeitlin11 @jbrockmendel 