Add a test encoder taking LZ matches as input

[gendx]

The current dumb encoder only generates a sequence of byte literals to be encoded one by one. This makes it hard to unit test decoding of LZ matches (distance + length).

Although finding matches in real inputs is a non-trivial task (#9), there could be some "synthetic" encoder taking as input a sequence of already prepared matches.

Something like the following.

enum LZMAElement {
    Literal(u8),
    Match(usize, usize),
}

fn compress(input: impl Iterator<Item=LZMAElement>) {
    /* TODO */
}