kem: README.md example isn't tested

[tarcieri]

In the kem crate's Cargo.toml there's the following:

[lib]
doctest = false

This is because the README.md is included in the rustdoc, except the example is more like pseudocode than an actually valid example.

A big problem with this is it isn't actually tested, which means breaking changes to the kem crate's API which need to be updated in the README.md aren't being caught in CI.