Proposed new Decoder option: set_readonly

[dams]

As per $subject, I've made a PR [1], but alos have been told to send an email here.

The idea is simple: give the possibility to set the whole output structure readonly, to avoid further tampering of it. For example, the structure can be passed to a set of user-defined plugins, with the (reasonable) assurance that each of them will see the original structure, and no plugin will accidentally change it.

I'm pretty sure the algorithm is crap (it keeps a reference on the SVs to set them readonly all at once at the end) and the implementation not better (XS newbie here). So I'm interested by any feedback.

Thanks,

dams.

[1]: https://github.com/Sereal/Sereal/pull/57