sagemathgh-37784: Speed up symmetric group simple_module_rank()

    
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Since we are going to work with a matrix $f^{\lambda} \times
f^{\lambda}$ (recall $f^{\lambda}$ is the number of standard Young
tableaux of shape $\lambda$), then we should just store the list of
standard tableaux rather than iterating over them repeatedly. This
results in a major speedup:
```
sage: from sage.combinat.specht_module import simple_module_rank
sage: %time simple_module_rank([5,3,2], GF(2))
CPU times: user 1.34 s, sys: 35.7 ms, total: 1.38 s
Wall time: 1.38 s
200
```
versus before
```
sage: %time simple_module_rank([5,3,2], GF(2))
CPU times: user 4.97 s, sys: 65 ms, total: 5.03 s
Wall time: 5.06 s
200
```

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URL: sagemath#37784
Reported by: Travis Scrimshaw
Reviewer(s): Darij Grinberg

src/sage/combinat/specht_module.py

@@ -1352,12 +1352,12 @@ def tabloid_gram_matrix(la, base_ring):
         [4 1 1 2 4]
     """
     from sage.combinat.tableau import StandardTableaux
-    ST = StandardTableaux(la)
+    ST = list(StandardTableaux(la))
 
     def bilinear_form(p1, p2):
         if len(p2) < len(p1):
             p1, p2 = p2, p1
-        return sum(c1 * p2.get(T1, 0) for T1, c1 in p1.items() if c1)
+        return sum(c1 * p2[T1] for T1, c1 in p1.items() if c1 and T1 in p2)
 
     PT = {T: polytabloid(T) for T in ST}
     gram_matrix = [[bilinear_form(PT[T1], PT[T2]) for T1 in ST] for T2 in ST]