Add Crystal Digest overrides

src/crypto-secret/crystal.cr

@@ -0,0 +1,13 @@
+abstract class Digest
+  def update(data : Crypto::Secret)
+    data.readonly do |slice|
+      update slice
+    end
+  end
+
+  def final(data : Crypto::Secret)
+    data.readwrite do |slice|
+      final slice
+    end
+  end
+end

src/crypto-secret/secret.cr

@@ -1,5 +1,6 @@
 require "./lib"
 require "./class_methods"
+require "./crystal"
 
 # Interface to hold sensitive information (often cryptographic keys)
 #