Use Map shim with _.uniq

[jridgewell]

Rounding out my PRs for today: An alternative implementation of #2099, using a Map shim. Performs just as well, but simplifies _.uniq instead of making it more complex.

This also paves the way for using a Map in _.memoize (#1862).

[megawac]

This also paves the way for using a Map in _.memoize (#1862).

This won't be possible (by default) as theres no reasonable way to shim a WeakMap so in older environments (such as node 0.10) we'll end up creating some potentially severe memory leaks

[megawac]

Why are you using Map instead of Set here?

See 2ce6f62

[jridgewell]

This won't be possible (by default)

It's just a Map, not a WeakMap. But this'll allow dev's to use a WeakMap if they want.

Why are you using Map instead of Set here?

I wanted to kill two birds with one stone. And, this is a direct port of #2099.

[jgonggrijp]

I think this would be the right thing to do in principle, but I hope it can be done in a way that takes up less space.