Update definition of returned iterator of _it_from of RB tree

from the greatest element of 'rbtree' lower of equal than 'data'
to the lowest element of the tree 'rbtree' greater or equal than 'data'

It is more logical and natural:
if you have a tree of elements between [2, 4], and you want
to iterate all elements from 0 to 10, the old definition doesn't allow
you to iterate easily over this subset.

tests/test-mrbtree.c

@@ -351,11 +351,6 @@ static void test_from(void)
   assert(!rbtree_uint_it_while_p(it, 2));
 
   rbtree_uint_it_from(it, tree, 2);
-  assert(rbtree_uint_end_p(it));
-  assert(rbtree_uint_it_until_p(it, 3));
-  assert(!rbtree_uint_it_while_p(it, 3));
-
-  rbtree_uint_it_from(it, tree, 4);
   assert(!rbtree_uint_end_p(it));
   assert(rbtree_uint_it_until_p(it, 3));
   assert(rbtree_uint_it_while_p(it, 3));
@@ -364,6 +359,11 @@ static void test_from(void)
   assert(rbtree_uint_it_until_p(it, 2));
   assert(!rbtree_uint_it_while_p(it, 2));
 
+  rbtree_uint_it_from(it, tree, 4);
+  assert(rbtree_uint_end_p(it));
+  assert(rbtree_uint_it_until_p(it, 3));
+  assert(!rbtree_uint_it_while_p(it, 3));
+
   // More elements
   rbtree_uint_push(tree, 10);
   rbtree_uint_push(tree, 5);
@@ -410,24 +410,32 @@ static void test_from(void)
   }
   assert(k == 2);
 
-  // No element is lower or equal than 0 ==> The set is empty
   k = 0;
+  const unsigned int tab1[] = { 1, 3, 5};
   for( rbtree_uint_it_from(it, tree, 0); rbtree_uint_it_while_p(it, 6); rbtree_uint_next(it)) {
     assert (k < 3);
-    assert(tab0[k] == *rbtree_uint_cref(it));
-    k++;
-  }
-  assert(k == 0);
-
-  // The iteration starts from 3 which is lower or equal than 4.
-  k = 0;
-  for( rbtree_uint_it_from(it, tree, 4); rbtree_uint_it_while_p(it, 7); rbtree_uint_next(it)) {
-    assert (k < 3);
-    assert(tab0[k] == *rbtree_uint_cref(it));
+    assert(tab1[k] == *rbtree_uint_cref(it));
     k++;
   }
   assert(k == 3);
 
+  k = 0;
+  const unsigned int tab2[] = { 5, 7};
+  for( rbtree_uint_it_from(it, tree, 4); rbtree_uint_it_while_p(it, 7); rbtree_uint_next(it)) {
+    assert (k < 2);
+    assert(tab2[k] == *rbtree_uint_cref(it));
+    k++;
+  }
+  assert(k == 2);
+
+  // From higher than all elements in the tree ==> no iterator.
+  k = 0;
+  for( rbtree_uint_it_from(it, tree, 11); rbtree_uint_it_while_p(it, 17); rbtree_uint_next(it)) {
+    assert (k < 2);
+    k++;
+  }
+  assert(k == 0);
+
   rbtree_uint_clear(tree);
 }