Function secp256k1::ffi::secp256k1_ec_pubkey_serialize

pub unsafe fn secp256k1_ec_pubkey_serialize(
    cx: *const Context, 
    output: *mut c_uchar, 
    out_len: *mut usize, 
    pk: *const PublicKey, 
    compressed: c_uint
) -> c_int

Serialize PublicKey back to 33/65 byte pubkey