Support I/O of the RNG state

[rstub]

With #57 all RNGs have >> and << operators. Making those available at the R level should enable some interesting applications, c.f. s3alfisc/fwildclusterboot#94 (comment)

Unfortunately, for the dqrng package, it is currently not possible to fetch the global seed state (as via .Random.seed for the 'standard R' random seed).

With some tests this would also help the currently quite bad test coverage.

CC: @s3alfisc