Rename bench_internal variables
Rename bench_internal variables

The _x and _y suffices are confusing; they don't actually correspond
to X and Y coordinates. Instead replace them with arrays.

peterdettman/secp256k1,afk11/secp256k1,evoskuil/secp256k1,bitcoin-core/secp256k1,evoskuil/secp256k1,afk11/secp256k1,afk11/secp256k1,bitcoin/secp256k1,evoskuil/secp256k1,evoskuil/secp256k1,ElementsProject/secp256k1-zkp,bitcoin/secp256k1,afk11/secp256k1,evoskuil/secp256k1,afk11/secp256k1,peterdettman/secp256k1,bitcoin/secp256k1,bitcoin/secp256k1,bitcoin-core/secp256k1,bitcoin/secp256k1,ElementsProject/secp256k1-zkp