Update feature gates

[ankurdubey521]

The original code was not building for me locally, with the following compiler errors:

error[E0635]: unknown feature `hash_drain_filter`
 --> src/lib.rs:5:12
  |
5 | #![feature(hash_drain_filter)]
  |            ^^^^^^^^^^^^^^^^^

error[E0635]: unknown feature `option_result_contains`
 --> src/lib.rs:4:12
  |
4 | #![feature(option_result_contains)]
  |            ^^^^^^^^^^^^^^^^^^^^^^

error[E0635]: unknown feature `drain_filter`
 --> src/lib.rs:3:12
  |
3 | #![feature(drain_filter)]
  |            ^^^^^^^^^^^^

1. Based on this and this, drain_filter has been renamed to extract_if and hash_drain_filter to hash_extract_if. Therefore I've substituted the drain_filter calls with equivalent extract_if calls.
2. Based on this, the option_result_contains has been removed, as far as I can see there are no substitues. The code builds without this gate so I've removed it, but I'm not completely sure if that's the correct thing to do.

[michaelsproul]

Great, thanks!

I think we should try to port the code away from the nightly compiler altogether. To use it in Lighthouse we need to be able to compile it with the stable compiler.

Updating all of the code might not be necessary, as we only need the Bron Kerbosch algorithm for the aggregation feature. We could port BK to a crate inside Lighthouse, which we can then tweak to further fit our needs. I suspect there'll be a few other changes required to the structure of the algorithm to make it work efficiently with our data types.