Remove individual copies of mbedtls_zeroize()

This commit removes all the static occurrencies of the function
mbedtls_zeroize() in each of the individual .c modules. Instead the
function has been moved to utils.h that is included in each of the
modules.
diff --git a/library/asn1parse.c b/library/asn1parse.c
index 4dd65c0..10ec3d8 100644
--- a/library/asn1parse.c
+++ b/library/asn1parse.c
@@ -28,6 +28,7 @@
 #if defined(MBEDTLS_ASN1_PARSE_C)
 
 #include "mbedtls/asn1.h"
+#include "mbedtls/utils.h"
 
 #include <string.h>
 
@@ -43,11 +44,6 @@
 #define mbedtls_free       free
 #endif
 
-/* Implementation that should never be optimized out by the compiler */
-static void mbedtls_zeroize( void *v, size_t n ) {
-    volatile unsigned char *p = (unsigned char*)v; while( n-- ) *p++ = 0;
-}
-
 /*
  * ASN.1 DER decoding routines
  */