fix(go,rs): nested async support

[rvolosatovs]

Encoding/decoding nested futures (e.g. future<future<T>>) (inconsistently) indexed the inner future with an index of 0.
For future<future<T>> sent at path x, inner, pending, future element would have been sent on x.0.
For example:
pending(pending(true)) would be sent as:
x: 0x00, 0x00
x.0: 0x01

WIth this PR:
x: 0x00, 0x00, 0x01

This ensures the implementation is consistent with other types (e.g. results or options) and provides greater optimization opportunities (e.g. client-side data batching)

With the above behavior fixed and made consistent across Go and Rust, this also closes #266, which caused leaking subscriptions in Go using async. If not explicitly type-asserted for io.Closer and Close'd, the subscriptions will be closed using runtime finalizers