fix allocator corruption for large sizes (#7338)

* fix allocator corruption for large sizes
* allow large chunks to coalesce and added test case
* use correct constants in MaxBigChunkSize

tests/system/talloc2.nim

@@ -0,0 +1,37 @@
+const
+  nmax = 2*1024*1024*1024
+
+proc test(n: int) =
+  var a = alloc0(9999)
+  var t = cast[ptr UncheckedArray[int8]](alloc(n))
+  var b = alloc0(9999)
+  t[0] = 1
+  t[1] = 2
+  t[n-2] = 3
+  t[n-1] = 4
+  dealloc(a)
+  dealloc(t)
+  dealloc(b)
+
+# allocator adds 48 bytes to BigChunk
+# BigChunk allocator edges at 2^n * (1 - s) for s = [1..32]/64
+proc test2(n: int) =
+  let d = n div 256  # cover edges and more
+  for i in countdown(128,1):
+    for j in [-4096, -64, -49, -48, -47, -32, 0, 4096]:
+      let b = n + j - i*d
+      if b>0 and b<=nmax:
+        test(b)
+        #echo b, ": ", getTotalMem(), " ", getOccupiedMem(), " ", getFreeMem()
+
+proc test3 =
+  var n = 1
+  while n <= nmax:
+    test2(n)
+    n *= 2
+  n = nmax
+  while n >= 1:
+    test2(n)
+    n = n div 2
+
+test3()