SAS7BDAT parser: Faster string parsing
Speed up SAS7BDAT string reading.

Today this brings a modest 10% performance improvement. But together with the other changes I will be proposing it will be a major bottleneck.

- [ ] closes #xxxx (Replace xxxx with the Github issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

I've ran against all SAS7BDAT test files in the repo, here's the ASV results.

```
asv continuous main sas/rstrip -b sas
```

Run 1

```
       before           after         ratio
     [e915b0a4]       [e20cf659]
     <main>           <sas/rstrip>
-     8.13±0.03ms      5.02±0.02ms     0.62  io.sas.SAS.time_productsalessas7bdat
-      14.9±0.3ms      7.32±0.05ms     0.49  io.sas.SAS.time_load_logsas7bdat

SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
PERFORMANCE INCREASED.
```

Run 2

```
-      8.35±0.2ms      4.96±0.02ms     0.59  io.sas.SAS.time_productsalessas7bdat
-      14.8±0.2ms      7.29±0.02ms     0.49  io.sas.SAS.time_load_logsas7bdat
```

Shall I add those to the ASV suite (or simply all of the files)? thanks @jonashaag 