Add operators for references to bitsets

[Aceeri]

Forgot to include this in my previous PR.

[kvark]

looks fine to me, as long as we can still tell for sure which of the operator functions is called at any place

[torkleyy]

Please add xor and tests as requested by @WaDelma, then merge.

bors delegate=@Aceeri

[bors[bot]]

:v: Aceeri can now approve this pull request

[Aceeri]

bors r+

[bors[bot]]

Build succeeded

• continuous-integration/travis-ci/push