added simple example + fixed warning in ast.rs

[Cr0a3]

Hi,
I added a simple example (examples/simple.rs) which is just the readme example.
I also fixed the unused warning in src/ast.rs:359 by adding #[allow(unused)]
Bye

[Cr0a3]

@fitzgen

[khuey]

Thanks for the PR. As CI shows, this example needs to be disabled for the no_std build.

[khuey]

That's not sufficient. You need something like this in Cargo.toml

[[example]]
name = "simple"
path = "examples/simple.rs"
required-features = ["std"]

[Cr0a3]

I fixed it

[khuey]

Thanks!