GPT-Encoder

Rust BPE Encoder Decoder for GPT-2 / GPT-3

This is rewrite of openai's gpt-2 encoder and latitudegames's GPT-3-Encoder in rust

```rs use gpt_encoder::Encoder;

fn main() { let mut encoder = Encoder::new(); let encoded = encoder.encode("Hello, World".to_string()); println!("{:?}", encoded); // prints: [15496, 11, 2159]

let decoded = encoder.decode(encoded);
println!("{:?}", decoded); 
// prints: "Hello, World"

} ```