Introduce Interface helper to allow calling protocol functions

[udoprog]

This allows for rewriting functions to receive protocols as arguments, an example of this is the now rewritten Iterator::chain:

/// Chain this iterator with another.
pub fn chain(self, other: Interface) -> Result<Self, VmError> {
    let other = other.into_iter()?;

    Ok(Self {
         inner: Inner::Chain(Box::new(Chain {
             a: Some(self.inner),
             b: Some(other.inner),
        })),
    })
}

This used to receive other: Iterator, meaning the caller has to convert the argument into an iterator. This is now instead done in the caller.

This requires the addition of a thread-local and a little bit of unsafe. Without this, we would have to pass an optional environment into every vm call, which would be messy to do with external abstractions like Function. But it also means that external abstractions will error if an environment is not available.