Added Benchmarking Library

[wnats]

Added a library for benchmarking purposes, as requested by @pschachte.

This is similar to #304, but with @pschachte's suggestion to store the clock counter as a global variable in cbits.c.

This library introduces two new semipure procedures, !benchmark.start and !benchmark.end(?time:float), serving as a pair of probes to measure the CPU clock time elapsed between two points of a program. A utility procedure !benchmark.time_execution(proc:{semipure}(), ?time:float) is also introduced which times proc. The library will throw an error if:

• !benchmark.start is called twice without a !benchmark.end(?time:float) in between
• !benchmark.end(?time:float) is called when an un-ended !benchmark.start has not been called
• !benchmark.time_execution(...) is called when there is a preceding !benchmark.start which has not been ended

[jimbxb]

This is cool. I like that we can guarantee that the timer is only started and ended once in succession.

One request: can we add a utility of some kind like this?

def {semipure} time(proc:{semipure}(), ?time:float) {
    !start
    !proc
    !end(?time)
}

[pschachte]

Thank you!