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/field_impl.h

@@ -19,7 +19,7 @@
 #endif
 
 SECP256K1_INLINE static void secp256k1_fe_clear(secp256k1_fe *a) {
-    secp256k1_memclear(a, sizeof(secp256k1_fe));
+    secp256k1_memclear_explicit(a, sizeof(secp256k1_fe));
 }
 
 SECP256K1_INLINE static int secp256k1_fe_equal(const secp256k1_fe *a, const secp256k1_fe *b) {