Add a helper for compressing a slice into a vec

[casey]

A common use case is compressing a &[u8] into a Vec<u8> without doing any I/O. The current docs don't make it clear how to do this.

A google search turns up this forum post with this code snippet:

use std::io::Write;

pub fn compress(input: &[u8]) -> Vec<u8> {
    let mut writer = brotli::CompressorWriter::new(
        Vec::new(),
        4096,
        11,
        22);
    writer.write_all(input).unwrap();
    writer.into_inner()
}

Which is nontrivial to figure out. A helper functions in the root of the crate to compress a &[u8] into a Vec<u8> and to decompress a Vec<u8> into a &[u8] would help a lot with this. (Also, if these functions don't do I/O then perhaps they could be be infallible.)