Return the memory after scrypt to the OS.

Previously a runtime.GC was being invoked which forced it to release the
memory as far as the garbage collector is concerned, but the memory was
not released back to the OS immediatley.  This modification allows the
memory to be released immedately since it won't be needed again until the
next wallet unlock.

snacl/snacl.go

@@ -6,7 +6,7 @@ import (
 	"encoding/binary"
 	"errors"
 	"io"
-	"runtime"
+	"runtime/debug"
 
 	"golang.org/x/crypto/nacl/secretbox"
 	"golang.org/x/crypto/scrypt"
@@ -129,7 +129,7 @@ func (sk *SecretKey) deriveKey(password *[]byte) error {
 	// example, if your scrypt parameters are such that you require 1GB and
 	// you call it twice in a row, without this you end up allocating 2GB
 	// since the first GB probably hasn't been released yet.
-	runtime.GC()
+	debug.FreeOSMemory()
 
 	return nil
 }