Reduce the number of rounds needed to create an incremental DPF

At each layer, we can create the next internal layer and the leaf layer
in parallel coroutines.

rdpf.tcc

@@ -883,29 +883,44 @@ RDPF<WIDTH>::RDPF(MPCTIO &tio, yield_t &yield,
         // The bit-shared choice bit is bit (depth-level-1) of the
         // XOR-shared target index
         RegBS bs_choice = target.bit(depth-level-1);
-        bool cfbit;
 
+        // At each layer, we can create the next internal layer and the
+        // leaf layer in parallel coroutines if we're making an
+        // incremental RDPF.  If not, exactly one of these coroutines
+        // will be created, and we just run that one.
+        std::vector<coro_t> coroutines;
         if (level < depth-1) {
-            DPFnode CW;
-            // This field is ignored when we're not expanding to a leaf
-            // level, but it needs to be an lvalue reference.
-            int noleafinfo = 0;
-            create_level(tio, yield, curlevel, nextlevel, player, level,
-                depth, bs_choice, CW, cfbit, save_expansion, noleafinfo,
-                aes_ops);
-            cfbits |= (value_t(cfbit)<<level);
-            if (player < 2) {
-                cw.push_back(CW);
-            }
+            coroutines.emplace_back([this, &tio, curlevel, nextlevel,
+                player, level, depth, bs_choice, save_expansion,
+                &aes_ops] (yield_t &yield) {
+                    DPFnode CW;
+                    bool cfbit;
+                    // This field is ignored when we're not expanding to a leaf
+                    // level, but it needs to be an lvalue reference.
+                    int noleafinfo = 0;
+                    create_level(tio, yield, curlevel, nextlevel, player, level,
+                        depth, bs_choice, CW, cfbit, save_expansion, noleafinfo,
+                        aes_ops);
+                    cfbits |= (value_t(cfbit)<<level);
+                    if (player < 2) {
+                        cw.push_back(CW);
+                    }
+                });
         }
         if (incremental || level == depth-1) {
-            LeafNode CW;
-            create_level(tio, yield, curlevel, leaflevel, player, level,
-                depth, bs_choice, CW, cfbit, save_expansion,
-                li[depth-level-1], aes_ops);
-            leaf_cfbits |= (value_t(cfbit)<<(depth-level-1));
-            li[depth-level-1].leaf_cw = CW;
+            coroutines.emplace_back([this, &tio, curlevel, leaflevel,
+                player, level, depth, bs_choice, save_expansion,
+                &aes_ops](yield_t &yield) {
+                    LeafNode CW;
+                    bool cfbit;
+                    create_level(tio, yield, curlevel, leaflevel, player,
+                        level, depth, bs_choice, CW, cfbit, save_expansion,
+                        li[depth-level-1], aes_ops);
+                    leaf_cfbits |= (value_t(cfbit)<<(depth-level-1));
+                    li[depth-level-1].leaf_cw = CW;
+                });
         }
+        run_coroutines(yield, coroutines);
 
         if (!save_expansion) {
             delete[] leaflevel;