Add `Throughput::Bits` or equivilent

[howardjohn]

This is honestly pretty nitpicky.. but it would be nice to be able to report in Bits instead of Bytes. Its fairly common in networking to report in bits, so I find myself mentally *8ing results quite a bit. It could be nice to have a Throughput::Bits.

one thing I thought of though is to pass Throughput::Bits(bits: usize) may be a bit odd; general bits aren't stored, but bytes. So you would really probably want Throughput::Bits(bytes: usize) which is even more confusing. So I suppose if this was implemented Throughput::Bits(bytesToWrite * 8) would be fairly common which I guess is fine.

(It says a lot about how great criterion is that this is my only complaint :slightly_smiling_face: )