Use Thread instance variables to handle XML errors

Use instead of ThreadLocal variables which don't play well with the GC and they
are not supported in all platforms. Thanks @asterite, @RX14 and @ysbaddaden!

src/xml/error.cr

@@ -11,49 +11,59 @@ class XML::Error < Exception
     super(message)
   end
 
-  @[ThreadLocal]
-  @@errors = [] of self
+  def self.init_thread_error_handling
+    Thread.current.xml_init_error_handling
+  end
 
-  @[ThreadLocal]
-  @@initialized = false
+  # :nodoc:
+  def self.set_errors(node)
+    if errors = self.errors
+      node.errors = errors
+    end
+  end
 
-  def self.init_thread_error_handling
-    return if @@initialized
+  def self.errors
+    xml_errors = Thread.current.xml_errors
+    if xml_errors.empty?
+      nil
+    else
+      errors = xml_errors.dup
+      xml_errors.clear
+      errors
+    end
+  end
+end
+
+class Thread
+  @__xml_errors = [] of XML::Error
+  @__xml_errors_initialized = false
 
-    LibXML.xmlSetStructuredErrorFunc nil, ->(ctx, error) {
-      @@errors << XML::Error.new(error)
+  def xml_errors
+    @__xml_errors
+  end
+
+  def xml_init_error_handling
+    return if @__xml_errors_initialized
+
+    LibXML.xmlSetStructuredErrorFunc self.as(Void*), ->(ctx, error) {
+      thread = ctx.as(Thread)
+      thread.xml_errors << XML::Error.new(error)
     }
 
-    LibXML.xmlSetGenericErrorFunc nil, ->(ctx, fmt) {
+    LibXML.xmlSetGenericErrorFunc self.as(Void*), ->(ctx, fmt) {
+      thread = ctx.as(Thread)
       # TODO: use va_start and va_end to
       message = String.new(fmt).chomp
       error = XML::Error.new(message, 0)
 
       {% if flag?(:arm) || flag?(:aarch64) %}
         # libxml2 is likely missing ARM unwind tables (.ARM.extab and .ARM.exidx
         # sections) which prevent raising from a libxml2 context.
-        @@errors << error
+        thread.xml_errors << error
       {% else %}
         raise error
       {% end %}
     }
-    @@initialized = true
-  end
-
-  # :nodoc:
-  def self.set_errors(node)
-    if errors = self.errors
-      node.errors = errors
-    end
-  end
-
-  def self.errors
-    if @@errors.empty?
-      nil
-    else
-      errors = @@errors.dup
-      @@errors.clear
-      errors
-    end
+    @__xml_errors_initialized = true
   end
 end