Rethink DUK_ENUM_NO_PROXY_BEHAVIOR flag

[svaarala]

In Duktape 2.2 Proxy objects have an internal duk_hproxy structure and the target/handler properties are no longer visible as hidden Symbols on the proxy. As a result, duk_enum() with DUK_ENUM_NO_PROXY_BEHAVIOR always returns an empty object when the target is a Proxy. This may be useful if one simply wants to avoid Proxy side effects when enumerating, but:

• If the goal is to avoid side effects when enumerating, one would also need the ability to avoid triggering getters but still have the ability to read plain property values.
• As a general use case inspection is a much wider issue than just enumeration, so perhaps side effect free inspection of objects should be possible without duk_enum().

[svaarala]

Some related discussion in https://github.com/svaarala/duktape/issues/1592#issuecomment-328233511.