[Rust] Offer a way to memcpy slices of integers to the destination

[mstange]

use flexbuffers::Builder;
let mut builder = Builder::default();
let mut root = builder.start_map();
root.push("my_ints", &[1i32, 2, 3, 4, 5]);
root.end_map();
write_to_file(builder.view());

Here, pushing the &[i32] iterates over all integers in the slice, finds the minimal width that's needed to represent all values, converts all values to that size, and writes them as little-endian.

Is there a more efficient way which boils down to a memcpy? My slices are very large and I would like to write them to the destination buffer as fast as possible. I don't mind if the stored representation isn't as compact as it could be.