Improving benchmarks

[amirouche]

A phase ahead, here are the things I would like to improve when #11 will be done, in order:

• compile Chez scheme programs to address https://github.com/ecraven/r7rs-benchmarks/issues/48
• compare cffi overhead to address https://github.com/ecraven/r7rs-benchmarks/issues/52!
• replace cpu limit with a wall clock time limit
• time the compilation step
• measure memory for compilation step
• measure memory for run step
• rework the graphing tool
• provide a safe and unsafe compilation option to address https://github.com/ecraven/r7rs-benchmarks/issues/43