&parse-at cannot be used with external stream

[bbannier]

We currently explicitly disallow in the documentation to use &parse-at with an external stream. This is not symmetric with &parse-from which allows external data. I think it would make sense to allow using &parse-at with an external stream so users can directly control the stream (something which we do not allow for the usual input stream).

Properly supporting this probably requires correct tracking of current unit iterator (begin and position).

Currently the following fails (like documented):

module foo;

global s = stream(b"\x00\x01\x02");

public type X = unit {
    x: uint8 &parse-at=s.at(0);
};

$ echo | spicy-dump foo.spicy
[fatal error] terminating with uncaught exception of type hilti::rt::InvalidIterator: incompatible iterator (foo.spicy:6:8)