add Set#from_yaml

spec/std/yaml/serialization_spec.cr

@@ -77,6 +77,10 @@ describe "YAML serialization" do
       Array(Int32).from_yaml("---\n- 1\n- 2\n- 3\n").should eq([1, 2, 3])
     end
 
+    it "does Set#from_yaml" do
+      Set(Int32).from_yaml("---\n- 1\n- 2\n- 2\n").should eq(Set.new([1, 2]))
+    end
+
     it "does Array#from_yaml from IO" do
       io = IO::Memory.new "---\n- 1\n- 2\n- 3\n"
       Array(Int32).from_yaml(io).should eq([1, 2, 3])

src/yaml/from_yaml.cr

@@ -89,6 +89,22 @@ def Array.new(ctx : YAML::ParseContext, node : YAML::Nodes::Node)
   end
 end
 
+def Set.new(ctx : YAML::ParseContext, node : YAML::Nodes::Node)
+  ctx.read_alias(node, self) do |obj|
+    return obj
+  end
+
+  unless node.is_a?(YAML::Nodes::Sequence)
+    node.raise "Expected sequence, not #{node.class}"
+  end
+
+  set = new
+  node.each do |value|
+    set << T.new(ctx, value)
+  end
+  set
+end
+
 def Hash.new(ctx : YAML::ParseContext, node : YAML::Nodes::Node)
   ctx.read_alias(node, self) do |obj|
     return obj