Fix namespaced vs top-level type-name collisions

Allow top-level property types to have the same name as DB-namespaced
ones.

spec/serializable_spec.cr

@@ -104,6 +104,23 @@ struct ModelWithEnum
   end
 end
 
+# Ensure types that happen to have the same name as types within the `DB`
+# namespace don't clash here
+struct Transaction
+  include DB::Serializable
+
+  @[DB::Field(key: "c0")]
+  getter id : Int32
+  @[DB::Field(key: "c1")]
+  getter status : Status
+
+  enum Status
+    Pending
+    Complete
+    Canceled
+  end
+end
+
 macro from_dummy(query, type)
   with_dummy do |db|
     rs = db.query({{ query }})
@@ -206,6 +223,13 @@ describe "DB::Serializable" do
     end
   end
 
+  it "should compile when top-level names collide with DB-namespaced names" do
+    expect_model("1,Pending", Transaction, {
+      id:     1,
+      status: Transaction::Status::Pending,
+    })
+  end
+
   it "should initialize multiple instances from a single resultset" do
     with_dummy do |db|
       db.query("1,a 2,b") do |rs|

src/db/serializable.cr

@@ -136,7 +136,7 @@ module DB
                     {% elsif value[:nilable] || value[:default] != nil %}
                       rs.read(::Union({{value[:type]}} | Nil))
                     {% else %}
-                      rs.read({{value[:type]}})
+                      rs.read(::{{value[:type]}})
                     {% end %}
                 rescue exc
                   ::raise ::DB::MappingException.new(exc.message, self.class.to_s, {{name.stringify}}, cause: exc)
@@ -166,7 +166,7 @@ module DB
           {% elsif value[:default] != nil %}
             @{{key}} = %var{key}.is_a?(Nil) ? {{value[:default]}} : %var{key}
           {% else %}
-            @{{key}} = %var{key}.as({{value[:type]}})
+            @{{key}} = %var{key}.as(::{{value[:type]}})
           {% end %}
         {% end %}
       {% end %}