feat: implement proxies support

[x1unix]

This PR adds support of proxies. Proxies allow intercepting property access, like Python's __get__ magic method.

Proxies allow implementing different patterns like prototype chaining, read on demand, etc.

Fixes: #777

[antonmedv]

Relying only on real time to check for proxy interface is not the best strategy. We need to do a pre-optimization and check at compile time whether a special opcode for fetching using this interface can be inserted ahead of time. With just a runtime check for proxy, I cannot accept this pull request.

[x1unix]

I've a couple of questions:

• By ahead of time optimization, do you mean to implement env check in optimizer.Optimize method?
  • Current API allows using different envs for parsing, optimization and evaluation.
    This means that during eval stage, user can supply an env that may have a proxy (e.g. foo.bar.baz), but this wasn't accounted during parse/optimize stage, so iface check still might be required inside OpFetch.
  • Proxy may be located deeply inside env (or populated at runtime), or returned from a func.
  • Initially, I wanted to use type check instead of an interface as it is 2x cheaper, but couldn't find a suitable approach (see benchmark). Happy to accept different proposals.
  • As 90% of users probably won't need proxies, this feature can be opt-in (feature flag). This will keep perf the same as before and will skip proxy checks by default.