Provide helper for parser

[KazDragon]

At the moment, parser only knows about vectors. And it should stay that way.

It needs a helper, though, since telnetpp will require a stream of (vector | any), and those anys must be passed through.

Therefore, we need a parser helper that accepts the compound type and returns a vector, where all the vectors have been buffered up and parsed into elements, and all the anys have been passed through.

Note: the anys must be passed through as close to their reception point as necessary. It's expected that these will always occur on element boundaries within the streams, but it's not necessarily true.

[KazDragon]

After some thought, it's probably not for the best that the interface is being weighed down so. It's getting little cumbersome. So it may be better to revert generate to trafficking only in vectors, and to introduce a "telnetpp::session" object, which can encapsulate all of the routing, parsing and generating behind three functions: install(option), receive(vector) ->vector and send(vector) -> vector.

Except even that looks a little lopsided with tokens on one side and elements on the other. Problem is that receiving data returns an immediate response that should be sent to the lower layer. The same isn't true of generating bytes for the lower layer with send.

[KazDragon]

I don't think I'll be able to get perfect symmetry out of this.