feat: add context manager

[rhkarls]

A context manager to time blocks of code, storing the results with some user passed identifier string

e.g.

with BenchmarkTimer("calculating something") as bt:
    a = calculate_something()
    b = a**2

prints something like Elapsed time to execute code block 'calculate something': 55 ms

bt stores the results, keeping the "calculating something" info, but its optional use as bt.