new egg: srfi-271

This library contains deterministic RNGs (and the system entropy source) 
wrapped in custom ports. This means that one can use the port IO 
procedures to read bytes from them. The deterministic RNGs have 
serializable state. RNGs can be seeded from other binary input ports.

Available algorithms:

* forward-secure chacha20 using fast key erasure
* non-forward-secure chacha20 that allows seeking to 512 byte blocks
* xoshiro256++
* splitmix64

Documentation: https://wiki.call-cc.org/eggref/6/srfi-271 (Not finished 
yet. Refer to SRFI 271 for usage.)

release-info: 
https://files.mcgoron.com/chicken/srfi-271-egg/srfi-271.release-info

-- Peter McGoron