PERF: consolidate imports inside parse_time_string
This PR adds an asv benchmark for the creation of `Period` objects directly from integers, and an associated speedup for the same case.

When calling `Period._from_ordinal()`, the runtime is unfortunately dominated by a pair of import statements needed to pull in the global config settings `display.date_dayfirst` and `display.date_yearfirst`. Thus, simply consolidating them leads to a significant speedup:
```
Benchmarks that have improved:

       before           after         ratio
     [08395af4]       [696b40f1]
     <parse_time_string~1>       <parse_time_string>
-         176±3ms         98.7±3ms     0.56  period.PeriodIndexConstructor.time_from_ints('D')
```

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
