refactor: Add return type to builder methods (#41)

src/blueprint/html/builder.cr

@@ -1,27 +1,23 @@
 module Blueprint::HTML
   # EXPERIMENTAL
-  def self.build(&)
-    Builder.build do |builder|
-      with builder yield
-    end
+  def self.build(&) : String
+    Builder.build { |builder| with builder yield }
   end
 
   private struct Builder
     include Blueprint::HTML
 
-    def self.build(&)
+    def self.build(&) : String
       builder = new
-      builder.build do
-        yield builder
-      end
+      builder.build { yield builder }
     end
 
-    private def blueprint(&)
-      yield
+    def build(&) : String
+      to_html { with self yield }
     end
 
-    protected def build(&)
-      to_html { with self yield }
+    private def blueprint(&) : Nil
+      yield
     end
   end
 end