PropertyKey does not accurately check conversions from Strings

[aapoalas]

We rely on an assumption that number-like PropertyKeys show up as Integer and that property keys can be compared by-value so that eg. "0" == 0 but the From and TryFrom implementations of PropertyKey do not actually check if strings are number-like.

As a result, ProperyKey::from("0") != PropertyKey::from(0) which is of course not correct.

This sort of points to the direction that the From<Value> and From<String> type of conversions for PropertyKey are invalid: We cannot freely and cheaply convert a string into a PropertyKey without first checking its contents. It would thus be better to remove these String and Value conversions and instead implement functions that do the same but also take an &Agent parameter so as to be capable of checking the string data.