Enable CI and write some tests that execute with wasmtime

[pchickey]

Built on top of #8

• Take the tcp_echo_server example and extract it into its own file in examples/ so we can include it in the docs and in the test-programs crate.
• Mark the echo server no_run so that cargo doc doesnt try to execute it.
• Re-enable CI running tests.
• Add a cargo workspace
• Introduce the same test-programs / artifacts arrangement we use in wasmtime. Two new crates, test-programs and test-programs-artifacts.
• tests/test-programs.rs executes the built artifacts with wasmtime, including external stimulus to the echo server example.
• add another test showing the http client working.