Add benchmarks and runner tool.

There are two benchmarks:
 - Basic: a simple loop with an arbitrary number of iterations and a single
   tracepoint.
 - Sha2: calculating an sha-256 hash on a 1MB buffer, with many tracepoints.

In both cases, we measure:
 - The time between the exevc() call and the beginning of main()
 - The run time of the algorithm

These measurements are taken in three scenarios:
 - Without linking with LTTng-UST at all
 - Linking with LTTng-UST but with tracing disabled
 - Tracing enabled

The benchmarks.py Python script runs the benchmarks and writes the results
in JSON format to benchmarks.json and in a Java properties-like format
in benchmarks.properties.