CreateAttachment::to_base64 causes slowdowns with big files

[Vrtgs]

I'm questionably trying to make discord store some big files, by splitting them and sending in chunks, and it seems there are slowdowns esspecially on debug builds, because sernity first turns the bytes into base64, and then inserts the string, which causes a realloc and a memcopy of some pretty large sections of memory (and this already happens a lot, because I have to split files, and then pass them to create attachment, which is another copy...... yeah this runs really slowly with big files on my old laptop)

and yes, I have profiled the code, this just seemed like an odd hotspot and an easy fix

anyways this function just seems easier and less of a breaking change than turning CreateAttachment

    pub fn to_base64(&self) -> String {
        let mut encoded = {
            use base64::Engine;
            base64::prelude::BASE64_STANDARD.encode(&self.data)
        };
        encoded.insert_str(0, "data:image/png;base64,");
        encoded
    }

I suggest an adimtedly much less simple implementation more like:

    pub fn to_base64(&self) -> String {
        use base64::Engine;

        const PREFIX: &str = "data:image/png;base64,";
        
        let engine = base64::prelude::BASE64_STANDARD;
        let encoded_len = base64::encoded_len(
            self.data.len(), 
            engine.config().encode_padding()
        )
            .and_then(|len| len.checked_add(PREFIX.len()))
            // we can't even allocate this much
            .expect("capacity overflow");
        
        let mut encoded = String::with_capacity(encoded_len);
        encoded.push_str(PREFIX);
        engine.encode_string(&self.data, &mut encoded);
        encoded
    }