size_test: add parseSize benchmark
size_test: add parseSize benchmark

Shows on my laptop (6 iterations processed by benchstat):

name         time/op
ParseSize-4  10.6µs ± 3%

name         alloc/op
ParseSize-4  3.26kB ± 0%

name         allocs/op
ParseSize-4    72.0 ± 0%

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

docker/go-units