Add back union and structure sigs

hearth/sig/lib/hearth/structure.rbs

@@ -0,0 +1,18 @@
+module Hearth
+  # A module mixed into Structs that provides utility methods.
+  module Structure
+    # Deeply converts the Struct into a hash. Structure members that
+    # are `nil` are omitted from the resultant hash.
+    #
+    # @return [Hash]
+    def to_h: (?untyped obj) -> untyped
+
+    alias to_hash to_h
+
+    private
+
+    def _to_h_struct: (untyped obj) -> untyped
+
+    def _to_h_hash: (untyped obj) -> untyped
+  end
+end

hearth/sig/lib/hearth/union.rbs

@@ -0,0 +1,6 @@
+module Hearth
+  # Top level class for all Union types
+  class Union < ::SimpleDelegator
+    include Hearth::Structure
+  end
+end