Non-allocating JavaScript object access

[AnsisMalins]

I already know you want me to somehow ensure that the struct does not escape the scope of the callback, so I'll work on that. But I'm sure there are other parts of this pull request you will have comments about. I'm unhappy about the V8Value.FastArgs, but I did not have better ideas so far.

Usage:

V8Object person = args.GetV8Object(0); // args is V8FastArgs
V8Value.FastArgs storage = default; // V8FastArg.pArg will point to this
string email = person.GetProperty("email", ref storage).GetString(); // GetProperty returns a V8FastArg
int age = person.GetProperty("age", ref storage).GetInteger();