Add Delete function

[chocolacula]

I believe checking index and bounds is better than keep it without the check and panicking with a bit weird error.

[chocolacula]

@elliotchance any thoughts?

[chocolacula]

I don't like the way it uses an output parameter. It should return a new slice not containing the element. Also, Delete a more common term used by languages when deleting something by it's key. And finally, are you sure this works with a single element? It looks like it would panic.

Done. One element will be deleted correctly. Added a test for that case.

[chocolacula]

I tried a few approaches and chose the best one. I added a little benchmark:

BenchmarkDelete-8                   3372           355714 ns/op          8003676 B/op      3 allocs/op
BenchmarkDelete-8                   2890           357530 ns/op          8003675 B/op      3 allocs/op
BenchmarkDeleteShort-8          14604861           80.06 ns/op           176 B/op          3 allocs/op
BenchmarkDeleteShort-8          14294144           80.04 ns/op           176 B/op          3 allocs/op
BenchmarkDeleteSet-8                 184           6437496 ns/op         8003588 B/op      1 allocs/op
BenchmarkDeleteSet-8                 184           6431757 ns/op         8003589 B/op      1 allocs/op
BenchmarkDeleteSetShort-8        8599725           137.4 ns/op           128 B/op          1 allocs/op
BenchmarkDeleteSetShort-8        8649344           137.3 ns/op           128 B/op          1 allocs/op

You can find them in separate branch in delete_test.go

[chocolacula]

@elliotchance please review