add missing interface methods on ABCs

These methods are present in the docs but not the actual interfaces.
diff --git a/cryptography/hazmat/primitives/interfaces.py b/cryptography/hazmat/primitives/interfaces.py
index 682a36d..33af438 100644
--- a/cryptography/hazmat/primitives/interfaces.py
+++ b/cryptography/hazmat/primitives/interfaces.py
@@ -191,6 +191,12 @@
         Returns an AsymmetricSignatureContext used for signing data.
         """
 
+    @abc.abstractmethod
+    def decrypt(self, ciphertext, padding):
+        """
+        Decrypts the provided ciphertext.
+        """
+
     @abc.abstractproperty
     def key_size(self):
         """
@@ -220,6 +226,12 @@
         Returns an AsymmetricVerificationContext used for verifying signatures.
         """
 
+    @abc.abstractmethod
+    def encrypt(self, plaintext, padding):
+        """
+        Encrypts the given plaintext.
+        """
+
     @abc.abstractproperty
     def key_size(self):
         """