replace dependency on assert

[chrisguttandin]

Hi @audioplastic,

thanks for providing this library. I'm a happy user for quite some time. I'm using ooura in the browser and I use webpack to bundle that the code. Up to v4 webpack did automatically include polyfills for the packages which are default packages of Node.js when building bundles for the browser. That's no longer the case in v5 which means ooura can't be used anymore without jumping through some hoops.

Would you welcome a PR which replaces assert with some inline checks?