Add symlinks to license files

[Pat-Lafon]

Closes #600

I don't fully understand the license implications here but from my reading, the correct solution from cargo has not fully manifested yet. In looking what other projects like tokio/regex do, they include the license files in each project so it seems reasonable to follow that strategy.

See bytecodealliance/wasmtime#4664 tokio-rs/tracing#842 rust-lang/cargo#8537 for some discussion on these issues.

[dburgener]

I read the associated links, and it seems like this is the right thing to do. Ideally, cargo would have support for copying this automatically, but since they've opted against doing that, it seems like this is what we need to do in order to meet our obligations under the apache and MIT licenses.

The biggest problem with duplication in software repos IMO is the risk of missing a copy when changing things. In the case of symlinks, this is only a problem if we move or rename our license files, which I imagine is something that would happen very infrequently.