Adding demonstrative test of O(n^2) string split case

Adding demonstrative test of O(n^2) string split case

Summary:
Even though the benchmark only keeps the first 10 lines, it gets slower the bigger the "file" is.

```
folly/gen/test/StringBenchmark.cpp              relative  time/iter  iters/s
----------------------------------------------------------------------------
Lines_Gen(1e3)                                               5.53us  180.94K
Lines_Gen(2e3)                                    66.43%     8.32us  120.21K
Lines_Gen(3e3)                                    48.26%    11.45us   87.33K
```

Test Plan: Benchmarks

Reviewed By: lesha@fb.com

Subscribers: folly-diffs@, yfeldblum

FB internal diff: D1823545

Tasks: 6155600

Signature: t1:1823545:1423023712:86fdb3dadbec44195e4b3596cf793cea80ae3a76