Add internal helper for serializing a non-infinity group element to a compressed public key

[theStack]

In more recent modules (ellswift, musig and silentpayments), the following pattern of serializing a non-infinity group element to a 33-bytes compressed public key appears repeatedly:

    size_t ser_size = 33;
    int ser_ret;
    .....
    ser_ret = secp256k1_eckey_pubkey_serialize(ge, out33, &ser_size, 1);
#ifdef VERIFY
    VERIFY_CHECK(ser_ret && ser_size == 33);
#else
    (void)ser_ret;
#endif

One could deduplicate these instances by a new function, as suggested by @w0xlt in the latest silentpayments module PR, see #1765 (review). This could simplify the code in the modules quite a bit (less LOC, less variables, less pre-processor blocks). Demo branch: theStack@e023651. It seems a reasonable idea to me, but not sure about the right naming of the function and where to put it. Also, one drawback might be that if the VERIFY_CHECK fails, it's a bit harder to find out the cause if it fails in a helper rather than directly in the module code (not sure if we care about that?). Opening this as an issue first to see if there is conceptual support.