Improve the ergonomics of registering host functions

[jprendes]

This PR adds the register and register_with_extra_allowed_syscalls methods to the UninitializedSandbox to register host methods.
e.g.,

uninitialized_sandbox.register("add", |a: i32, b: i32| {
    Ok(a + b)
})?;

This method accepts either:

• FnMut(..) -> Result<R>
• Arc<Mutex<FnMut(..) -> Result<R>>>
• &Arc<Mutex<FnMut(..) -> Result<R>>>

This is controlled by the IntoHostFunction trait.

This is not a breaking change. It's an additive change, and the previous registration mechanism continues to works.

[ludfjig]

lgtm!

[danbugs]

LGTM. Wonder if we should just remove the previous registration mechanism...

[jprendes]

Wonder if we should just remove the previous registration mechanism...

The new mechanism is a wrapper around the old one. I would need to think how we can stop exposing the old mechanism without violating some "X is more private than Y" rule.