[issue31415] Add -X option to show import time

[INADA Naoki]

INADA Naoki added the comment:

I don't have any good idea for it.

I usually run `python -X importtime -c 'import functools'` several times to ensure pyc files are created and it is cached by OS.

On Linux, `strace -c` and `time` command can be used.
But I don't know cross platform way to do it.

$ strace -c python -c 'import asyncio'
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
  0.00    0.000000           0       244           read
  0.00    0.000000           0       160         2 open
  0.00    0.000000           0       161           close
  0.00    0.000000           0       527        46 stat
  0.00    0.000000           0       266           fstat
  0.00    0.000000           0        16           lstat
  0.00    0.000000           0       114         6 lseek
  0.00    0.000000           0       109           mmap

$ /usr/bin/time python -c 'import asyncio'
0.12user 0.00system 0:00.13elapsed 98%CPU (0avgtext+0avgdata 17004maxresident)k
0inputs+0outputs (0major+2613minor)pagefaults 0swaps

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue31415>
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