Summary:
Running `infer --help` with spacetime showed that `Str` was a hog. And indeed perf is 74% better now:
```
before:
bash -c 'for i in $(seq 100); do infer --help > /dev/null 2>/dev/null; done' 28.02s user 0.62s system 98% cpu 29.205 total
after:
bash -c 'for i in $(seq 100); do infer --help > /dev/null 2>/dev/null; done' 6.34s user 0.64s system 92% cpu 7.557 total
```
Running on many files one after the other is now 64% faster too:
```
$ cd infer/tests/codetoanalyze/c/errors
$ # before
$ rm -fr infer-out && time bash -c 'for src in */*.c; do infer -a capture --continue -- clang -c $src; done'
bash -c 7.77s user 0.66s system 97% cpu 8.647 total
$ # after
$ rm -fr infer-out && time bash -c 'for src in */*.c; do infer -a capture --continue -- clang -c $src; done'
bash -c 2.35s user 0.56s system 93% cpu 3.119 total
$ time infer -a capture -- clang -c */*.c
infer -a capture -- clang -c */*.c 0.54s user 0.20s system 99% cpu 0.737 total
```
Reviewed By: mbouaziz
Differential Revision: D4875803
fbshipit-source-id: cfcfa69