.cursor implementation (#12637)

* cursors: first implementation
* added currently failing test
* .cursor works for doubly linked lists
* make -d:useMalloc work again
* added code to nil out refs in a destructor
* it's now called --gc:arc
* renderer.nim: render nkBreakState properly
* make simple closure iterators work without leaking

lib/core/allocators.nim

@@ -43,7 +43,7 @@ proc getLocalAllocator*(): Allocator =
     result = addr allocatorStorage
     result.alloc = proc (a: Allocator; size: int; alignment: int = 8): pointer {.nimcall, raises: [].} =
       when defined(useMalloc) and not defined(nimscript):
-        result = c_malloc(size)
+        result = c_malloc(cuint size)
         # XXX do we need this?
         nimZeroMem(result, size)
       else:
@@ -57,7 +57,7 @@ proc getLocalAllocator*(): Allocator =
       inc a.deallocCount
     result.realloc = proc (a: Allocator; p: pointer; oldSize, newSize: int): pointer {.nimcall, raises: [].} =
       when defined(useMalloc) and not defined(nimscript):
-        result = c_realloc(p, newSize)
+        result = c_realloc(p, cuint newSize)
       else:
         result = system.realloc(p, newSize)
       nimZeroMem(result +! oldSize, newSize - oldSize)

lib/core/runtime_v2.nim

@@ -47,7 +47,7 @@ proc nimNewObj(size: int): pointer {.compilerRtl.} =
   when defined(nimscript):
     discard
   elif defined(useMalloc):
-    var orig = c_malloc(s)
+    var orig = c_malloc(cuint s)
     nimZeroMem(orig, s)
     result = orig +! sizeof(RefHeader)
   else: