gh-35741: `sage.combinat`: Split some Cython modules (modularization fixes)
    
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### 📚 Description

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Some basic modules of `sage.combinat` (`.permutation`, `.combination`,
etc.) are needed in the distribution **sagemath-categories** (as of
#35095). We make them importable (by splitting a Cython module into
several parts and by using lazy imports) and separately testable (using
`# optional` doctest directives).

Likewise, split the parts of `sage.combinat.posets.hasse_cython` that
need the FLINT library out as a separate module (this is for the package
**sagemath-graphs** in #35095)

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- Split out from #35564
- Part of #29705
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- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.

### ⌛ Dependencies

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URL: #35741
Reported by: Matthias Köppe
Reviewer(s): David Coudert

Author: Release Manager

src/doc/en/reference/combinat/module_list.rst

@@ -291,6 +291,7 @@ Comprehensive Module List
     sage/combinat/rsk
     sage/combinat/schubert_polynomial
     sage/combinat/set_partition
+    sage/combinat/set_partition_iterator
     sage/combinat/set_partition_ordered
     sage/combinat/sf/all
     sage/combinat/sf/character

src/sage/combinat/combinat_cython.pxd

@@ -2,7 +2,4 @@ from sage.libs.gmp.all cimport mpz_t
 
 cdef mpz_stirling_s2(mpz_t s, unsigned long n, unsigned long k)
 
-cdef list from_word(list w, list base_set)
-
 cdef list convert(Py_ssize_t* f, Py_ssize_t n)
-