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.

[antonmedv]

By ahead of time optimization, do you mean to implement env check in optimizer.Optimize method?

The compiler. Compiler can produce optimized code.

Proxy may be located deeply inside env (or populated at runtime), or returned from a func.

Yes, in this case we can fallback to a common fetcher.

type check instead of an interface as it is 2x cheaper,

Even better to not do this at runtime at all!

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.

Yes, most of user will not need it. Idealy we hide this behide compier and do not expose additional checks at Fetch.