Skeletal draft of PRNG chapter

[nathanielknight]

This PR contains a skeletal draft of the PRNG chapter. My hope is that it's fleshed out enough for:

• sequencing decisions
• editorial feedback on the chapter based on what's covered elsewhere vs. what the chapter should spend wordcount on

I'd also very much appreciate any feedback on the code. I'm new to Roc and don't have much occasion for functional programming in my day job, so if I've implementing something in a conspicuously un-idiomatic way it's almost certainly out of ignorance and not for a deep editorial reason; please let me know. 😅

As it stands, the outline is for a chapter that would contain:

• a type definition for a generic PRNG
• a modest suite of tools that build on that type
• an implementation of RANDU, a very simple PRNG with awful statistical properties
• an illustration of RANDU's shortcomings built with the random tools
• a basic implementation of PCG, a fairly simple PRNG with much better statistical properties
• an illustration of PCG's superior output

Possible extensions (depending on space, what's covered elsewhere, etc.) include:

• additional random tools
• an implementation of rejection sampling to enable generating random elements from arbitrary probability distributions
• a quick survey of more quantitative tests of PRNG quality

[gvwilson]

I realize this is a work in progress, but may I merge it as-is so that I have some material when I start rearranging for order?

[nathanielknight]

Absolutely; I've been picking away at some prose, but it would be great to have a better idea of where this chapter will appear.