Rethink API for standard library features

[tertsdiepraam]

Roto doesn't enable the print function by default, because that might not be desirable. But the discoverability of that is not great right now. We should either enable it by default or make a better API for configuring which features of the standard library to enable.

Some ideas:

let std_lib = StandardLibrary::minimal();
Runtime::new(std_lib);

let std_lib = StandardLibrary::all();

let std_lib = StandardLibrary::new(StandardLibraryOptions {
    io: true,
    net: true,
    ..Default::default()
});

// short-hand
let rt = Runtime::minimal();