Expose Sodium::Sign::SecretKey#seed.

spec/sodium/sign/secret_key_spec.cr

@@ -52,6 +52,8 @@ describe Sodium::Sign::SecretKey do
     key2 = Sodium::Sign::SecretKey.new seed: seed
     key1.to_slice.should eq key2.to_slice
     key1.public_key.to_slice.should eq key2.public_key.to_slice
+    key1.seed.should eq seed
+    key1.seed.should eq key2.seed
   end
 
   it "signs and verifies" do

src/sodium/sign/secret_key.cr

@@ -70,6 +70,11 @@ module Sodium
       end
     end
 
+    def seed
+      # BUG: Generate seed if not set.
+      @seed.not_nil!.to_slice
+    end
+
     # Signs message and returns a detached signature.
     # Verify using `secret_key.public_key.verify_detached(message, sig)`
     def sign_detached(message)