Log-2 Axis Scale Plotting

[LebedevRI]

Allow to use log-2 base for logarithmically-scaled axis.
I've tested this, (separately from cargo-criterion change),
i believe this works as expected, although things could be better,
e.g. when using gnuplot, the ticks labels are sometimes outputted
in %e format, with base=10, which looks, perhaps, suboptimal.

[LebedevRI]

@berkus Hi! Do you have anything in particular in mind as to what changes are still needed here?
I will be perfectly happy to deal with any number of nits,
but i don't know what they are, because nothing really stands out to me here...

[LebedevRI]

<...>, although things could be better, e.g. when using gnuplot, the ticks labels are sometimes outputted in %e format, with base=10, which looks, perhaps, suboptimal.

So this is kind-of possible, #81,
but the more i look at plotting, the less i like existing backends.

Other than that, i'm not really aware of any pending touchups from my side.

[lemmih]

Looks fine. Can merge once the CHANGELOG has the entry under unreleased rather than under 0.8.2.

[berkus]

@LebedevRI please update the changelog line as @lemmih suggested and lets merge it!