Starting point for a benchmark to compare multiple-precision libs

[edyoung]

Uses Google benchmark library. Need to rearrange the docker stuff, it recompiles the benchmark library every time which slows things down a lot.

Run the benchmark with examples/cpp/mp_benchmark.sh

Sample output:

Running ./example
Run on (4 X 2700 MHz CPU s)
CPU Caches:
  L1 Data 32 KiB (x2)
  L1 Instruction 32 KiB (x2)
  L2 Unified 256 KiB (x2)
  L3 Unified 3072 KiB (x1)
Load Average: 2.38, 1.75, 1.11
***WARNING*** CPU scaling is enabled, the benchmark real time measurements may be noisy and will incur extra overhead.
***WARNING*** Library was built as DEBUG. Timings may be affected.
----------------------------------------------------------------------
Benchmark                            Time             CPU   Iterations
----------------------------------------------------------------------
BM_fractal/min_time:120.000       6823 ms         6823 ms           25

[mindhells]

This is very cool.
I've created #113 to update this branch:

• preventing the benchmark library to be recompiled every time
• update the examples readme

[mindhells]

bringing Boost into the project opens a world of possibilities, like:
https://github.com/boostorg/compute (in regard to #78)

[dragonmux]

I will warn that it can also severely complicate things - particularly when trying to get the project to build on a RHEL or SLES box because they invariably have very old Boost's and not installed by default even when you put the compilers on..

But as long as that's a trade-off you're willing to make, it is an intriguing idea