Implement browser getRandomValues without depending on crypto

This is based on this [discussion](https://github.com/browserify/crypto-browserify/issues/232)

TL;DR: I propose having an implementation of `getRandomValues` that does not depend on a native implementation of crypto.

I am having an issue with bundling my code and running it in a non-browser environment, using modules that depend on `crypto`, which depends on `randombytes`' implementation of `getRandomValues`. If I bundle my code in "browser" mode, I get the error `'Secure random number generation is not supported by this browser.\nUse Chrome, Firefox or Internet Explorer 11'`.

If I don't bundle it in browser mode but force browser fallback for `crypto-browserify` (which is necessary in my use-case, the javascript VM I'm using does not implement `crypto`), then I get a circular dependency and getRandomBytes is never implemented.

Is it possible to implement it in this package? If not is there any particular reason? And can I help in some way?

首先检查此 package 的 browser getRandomValues 实现以及链接的 crypto-browserify 讨论。跟踪在 browser 模式下如何选择 randombytes，以及 crypto-browserify fallback 如何创建所报告的循环依赖。当一个受支持的实现能够在所描述的非浏览器 JavaScript VM 中运行，且不依赖 native crypto 实现时，即表示完成。

由索引模型根据 Issue 内容生成。