Remove usage of CHECK from non-test file

Currently CHECK is used only in test and bench mark files except for one
usage in `ecmult_impl.h`.

We would like to move the definition of CHECK out of `util.h` so that
`util.h` no longer has a hard dependency on `stdio.h`.

Done as part of an effort to allow secp256k1 to be compiled to WASM
as part of `rust-secp256k1`.

src/ecmult_impl.h

@@ -199,12 +199,13 @@ static int secp256k1_ecmult_wnaf(int *wnaf, int len, const secp256k1_scalar *a,
 
         bit += now;
     }
-#ifdef VERIFY
-    CHECK(carry == 0);
+
+    VERIFY_CHECK(carry == 0);
     while (bit < 256) {
-        CHECK(secp256k1_scalar_get_bits(&s, bit++, 1) == 0);
+        VERIFY_CHECK(secp256k1_scalar_get_bits(&s, bit, 1) == 0);
+        bit++;
     }
-#endif
+
     return last_set_bit + 1;
 }