Split memclear into two versions

secp256k1_memclear has the side effect of undefining bytes for
valgrind checks. In some cases, we may want to zero bytes
but allow subsequent reads. So we split memclear into
memclear_explicit, which makes no guarantees about the content
of the buffer on return, and memzero_explicit, which guarantees
zero value on return.

Change the memset in partial_sign to use memzero_explicit.

src/modules/ellswift/main_impl.h

@@ -582,7 +582,7 @@ int secp256k1_ellswift_xdh(const secp256k1_context *ctx, unsigned char *output,
     /* Invoke hasher */
     ret = hashfp(output, sx, ell_a64, ell_b64, data);
 
-    secp256k1_memclear(sx, sizeof(sx));
+    secp256k1_memclear_explicit(sx, sizeof(sx));
     secp256k1_fe_clear(&px);
     secp256k1_scalar_clear(&s);