Deprecate td.function in favor of td.func

[searls]

Terrible luck to discover this after releasing 2.0.0, but subsequent improvements to the build have made me realize that the cute way we do this:

module.exports = { function: require('./function') /*...*/ }

Just won't fly in ES module-land, where something like this:

export { function }

Will blow up because it's a reserved word. (import {function} from 'testdouble' also blows up)

We can do:

export default {
  function: tdFunction
}

But then any module/bundler like webpack/rollup/browserify is going to have that default tacked onto it, which means that our distribution will be td.default.object or whatever.

I hope we can find some workaround to use ES exports exclusively and then solve this with build cleverness using one of the bundlers, but even if we do manage, having a top-level API that's a reserved word is probably not a great long-term play.

Goals:

• hack up some short-term win with a bundler that'll let us still get tree shaking while using a CJS export at the top level or a way to modify the bundled content to td = td.default dynamically.
• documentation change from td.function() to td.func()
• figure out how soon we want to think about dropping td.function with another major bump.