exp2, exp10, log2, log10 implimentation

[oscardssmith]

It is probably a good idea to use exp2 as the fundamental exponential function (it has fewer roundings) and then just implement exp and exp10 with an extra multiple by log2(e) and log2(10) respectively. A similar strategy can be used for logarithms.

[dzhang314]

Hey @oscardssmith , thanks for your interest in MultiFloats.jl! This is a great idea, as it simplifies the argument reduction step from subtracting an integer multiple of log(2) to simply subtracting an integer. I'll give this a shot, and if the benchmarks turn out favorably, you can expect to see this implemented in the next release.

[dzhang314]

Closing this issue to track in #28 instead.