WeakRef: Do not register non heap pointers as diasappearing links

This should allow at least pointers to the data section
to work seamlessly with WeakRef and in some cases even
allow stack pointers to be passed, for the crazy

spec/std/weak_ref_spec.cr

@@ -27,13 +27,25 @@ private class Foo
 end
 
 describe WeakRef do
-  it "should get dereference object" do
+  it "should get dereferenced object" do
     foo = Foo.new :foo
     ref = WeakRef.new(foo)
     ref.should_not be_nil
     ref.target.should be(foo)
   end
 
+  it "should get dereferenced object in data section" do
+    foo = "foo"
+    ref = WeakRef.new(foo)
+    ref.target.should be(foo)
+  end
+
+  it "should not crash with object in data section during GC" do
+    foo = "foo"
+    ref = WeakRef.new(foo)
+    GC.collect
+  end
+
   it "State counts released objects" do
     State.reset
     State.count(:foo).should eq 0

src/gc/boehm.cr

@@ -23,6 +23,7 @@ lib LibGC
   fun set_handle_fork = GC_set_handle_fork(value : Int)
 
   fun base = GC_base(displaced_pointer : Void*) : Void*
+  fun is_heap_ptr = GC_is_heap_ptr(pointer : Void*) : Int
   fun general_register_disappearing_link = GC_general_register_disappearing_link(link : Void**, obj : Void*) : Int
 
   type Finalizer = Void*, Void* ->
@@ -120,6 +121,10 @@ module GC
     LibGC.general_register_disappearing_link(pointer, base)
   end
 
+  def self.is_heap_ptr(pointer : Void*)
+    LibGC.is_heap_ptr(pointer) != 0
+  end
+
   record Stats,
     # collections : LibC::ULong,
     # bytes_found : LibC::Long,

src/weak_ref.cr

@@ -5,7 +5,9 @@ class WeakRef(T)
 
   def initialize(target : T)
     @target = target.as(Void*)
-    GC.register_disappearing_link(pointerof(@target))
+    if GC.is_heap_ptr(@target)
+      GC.register_disappearing_link(pointerof(@target))
+    end
   end
 
   def self.allocate