Overhaul benchmarking system

[unzvfu]

From data61#50:

Including, in no particular order

• Generate data and graph it, rather than generating large difficult-to-interpret tables.
• Generate data for all functions.
• Allow generating data for specific functions.
• Generate graphs of relative performance for related functions (e.g. How much faster is squaring to multiplication for various sizes?)
• For each operation, work out the appropriate amount of data to use (currently we use a tiny amount for small fixnums and a largish amount for bigger fixnums -- this is inconsistent).
• Understand what the optimal performance could be in important cases (e.g. mulmod, modexp).
• Normalise the data gathered relative to some appropriately chosen device parameters (e.g. #SMs, #cores, core frequency, ...)

[unzvfu]

Consider using the Google Benchmark library.