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/talloc.nim

@@ -0,0 +1,57 @@
+var x: ptr int
+
+x = cast[ptr int](alloc(7))
+assert x != nil
+x = cast[ptr int](x.realloc(2))
+assert x != nil
+x.dealloc()
+
+x = createU(int, 3)
+assert x != nil
+x.dealloc()
+
+x = create(int, 4)
+assert cast[ptr array[4, int]](x)[0] == 0
+assert cast[ptr array[4, int]](x)[1] == 0
+assert cast[ptr array[4, int]](x)[2] == 0
+assert cast[ptr array[4, int]](x)[3] == 0
+
+x = x.resize(4)
+assert x != nil
+x.dealloc()
+
+x = cast[ptr int](allocShared(100))
+assert x != nil
+deallocShared(x)
+
+x = createSharedU(int, 3)
+assert x != nil
+x.deallocShared()
+
+x = createShared(int, 3)
+assert x != nil
+assert cast[ptr array[3, int]](x)[0] == 0
+assert cast[ptr array[3, int]](x)[1] == 0
+assert cast[ptr array[3, int]](x)[2] == 0
+
+assert x != nil
+x = cast[ptr int](x.resizeShared(2))
+assert x != nil
+x.deallocShared()
+
+x = create(int, 10)
+assert x != nil
+x = x.resize(12)
+assert x != nil
+x.dealloc()
+
+x = createShared(int, 1)
+assert x != nil
+x = x.resizeShared(1)
+assert x != nil
+x.deallocShared()
+
+x = cast[ptr int](alloc0(125 shl 23))
+dealloc(x)
+x = cast[ptr int](alloc0(126 shl 23))
+dealloc(x)