Make SecretBuffer and shred! public

I'm assuming can and should (sometimes) be used, not just by Julia.

shred! ok in Strings? Maybe it or both should go under IO?

Author: PallHaraldsson

base/public.jl

@@ -13,6 +13,7 @@ public
     AbstractOneTo,
     AbstractPipe,
     AsyncCondition,
+    SecretBuffer,
     CodeUnits,
     Event,
     Fix,
@@ -108,6 +109,7 @@ public
 
 # Strings
     escape_raw_string,
+    shread!,
 
 # IO
     # types