Add a third size class (between small/large) to the gc.

port over sgen's idea of LOSSections as a mid-sized arena, so that we now have:

SmallArena original non-large allocator, free bitmaps, segregated-fit allocator.
  handles objects where size <= 3584 bytes
LargeArena (new code, size-specific free lists.)
  handles object  where 3584 < size <= ~1 meg
HugeArena (original large allocator, 1 mmap per object.
  handles objects where size > ~1meg

minibenchmarks/pidigits.py

+import time
+
+PIDIGITS_LEN = 1500
+
+def pidigits(length):
+    i = k = ns = 0
+    k1 = 1
+    n,a,d,t,u = 1,0,1,0,0
+    while(True):
+        k += 1
+        t = n<<1
+        n *= k
+        a += t
+        k1 += 2
+        a *= k1
+        d *= k1
+        if a >= n:
+            t,u = divmod(n*3 + a,d)
+            u += n
+            if d > u:
+                ns = ns*10 + t
+                i += 1
+                if i % 10 == 0:
+                    ns = 0
+                if i >= length:
+                    break
+                a -= d*t
+                a *= 10
+                n *= 10
+
+def main(n):
+    l = []
+    for i in range(n):
+        t0 = time.time()
+        pidigits(PIDIGITS_LEN)
+        l.append(time.time() - t0)
+    return l
+
+main(100)

test/unittests/gc.cpp

@@ -68,6 +68,9 @@ TEST(alloc, alloc64) { testAlloc(64); }
 TEST(alloc, alloc128) { testAlloc(128); }
 TEST(alloc, alloc258) { testAlloc(258); }
 TEST(alloc, alloc3584) { testAlloc(3584); }
+TEST(alloc, alloc4096) { testAlloc(4096); }
+TEST(alloc, alloc8192) { testAlloc(8192); }
+TEST(alloc, alloc16384) { testAlloc(16384); }
 
 TEST(alloc, largeallocs) {
     int s1 = 1 << 20;