Add timeit-loop

[primo-ppcg]

Related issue: #127

Adds a function which extends loop to output performance statistics after completion.

Sample usage:

(def ind (range 1000))
(timeit-loop [:timeout 1] "take" (take -500 ind))
(timeit-loop [:timeout 1] "drop" (drop 500 ind))
# ->
take 1.000s, 1.514µs/body
drop 1.000s, 1.496µs/body