Add rb_category_warn{,ing} for warning messages with categories

This adds the following C-API functions that can be used to emit
warnings with categories included:

```c
void rb_category_warn(const char *, const char*, ...)
void rb_category_warning(const char*, const char*, ...)
```

Internally in error.c, there is an rb_warn_category function
that will call Warning.warn with the string and the category
keyword if it doesn't have an arity of 1, and will call
Warning.warn with just the string if it has an arity of 1.
This refactors the rb_warn_deprecated{,_to_remove} functions
to use rb_warn_category.

This makes Kernel#warn accept a category keyword and pass it
to Warning.warn, so that Ruby methods can more easily emit
warnings with categories.  rb_warn_category makes sure that
the passed category is a already defined category symbol
before calling Warning.warn.

The only currently defined warning category is :deprecated,
since that is what is already used.  More categories can be
added in later commits.

core/kernel/warn_spec.rb

@@ -17,7 +17,7 @@
     Kernel.should have_private_instance_method(:warn)
   end
 
-  it "requires multiple arguments" do
+  it "accepts multiple arguments" do
     Kernel.method(:warn).arity.should < 0
   end
 
@@ -114,6 +114,38 @@
       end
     end
 
+    ruby_version_is "3.0" do
+      it "accepts :category keyword with a symbol" do
+        -> {
+          $VERBOSE = true
+          warn("message", category: :deprecated)
+        }.should output(nil, "message\n")
+      end
+
+      it "accepts :category keyword with nil" do
+        -> {
+          $VERBOSE = true
+          warn("message", category: nil)
+        }.should output(nil, "message\n")
+      end
+
+      it "accepts :category keyword with object convertible to symbol" do
+        o = Object.new
+        def o.to_sym; :deprecated; end
+        -> {
+          $VERBOSE = true
+          warn("message", category: o)
+        }.should output(nil, "message\n")
+      end
+
+      it "raises if :category keyword is not nil and not convertible to symbol" do
+        -> {
+          $VERBOSE = true
+          warn("message", category: Object.new)
+        }.should raise_error(TypeError)
+      end
+    end
+
     it "converts first arg using to_s" do
       w = KernelSpecs::WarnInNestedCall.new
 

core/warning/warn_spec.rb

@@ -51,14 +51,41 @@ def Warning.warn(msg)
     end
   end
 
-  it "is called by Kernel.warn" do
-    Warning.should_receive(:warn).with("Chunky bacon!\n")
-    verbose = $VERBOSE
-    $VERBOSE = false
-    begin
-      Kernel.warn("Chunky bacon!")
-    ensure
-      $VERBOSE = verbose
+
+  ruby_version_is '3.0' do
+    it "is called by Kernel.warn with nil category keyword" do
+      Warning.should_receive(:warn).with("Chunky bacon!\n", category: nil)
+      verbose = $VERBOSE
+      $VERBOSE = false
+      begin
+        Kernel.warn("Chunky bacon!")
+      ensure
+        $VERBOSE = verbose
+      end
+    end
+
+    it "is called by Kernel.warn with given category keyword converted to a symbol" do
+      Warning.should_receive(:warn).with("Chunky bacon!\n", category: :deprecated)
+      verbose = $VERBOSE
+      $VERBOSE = false
+      begin
+        Kernel.warn("Chunky bacon!", category: 'deprecated')
+      ensure
+        $VERBOSE = verbose
+      end
+    end
+  end
+
+  ruby_version_is ''...'3.0' do
+    it "is called by Kernel.warn" do
+      Warning.should_receive(:warn).with("Chunky bacon!\n")
+      verbose = $VERBOSE
+      $VERBOSE = false
+      begin
+        Kernel.warn("Chunky bacon!")
+      ensure
+        $VERBOSE = verbose
+      end
     end
   end
 end