Web circular dependency workaround

[buu700]

Quick fix for a circular dependency when using crypto-browserify with web module bundlers (in this case Angular CLI / webpack):

WARNING in Circular dependency detected: src/js/externals/crypto.ts -> node_modules/crypto-browserify/index.js -> node_modules/diffie-hellman/browser.js -> node_modules/diffie-hellman/lib/generatePrime.js -> node_modules/miller-rabin/lib/mr.js -> node_modules/brorand/index.js -> src/js/externals/crypto.ts

(Using eval this way feels a bit hackish, but I'm not aware of anything like Content Security Policy that would break it in a Node.js context, and I've been using this exact technique in all of @cyph's crypto libraries for a while now with no known issues.)