Building in ALT-REP to stringi

Have you considered implementing an ALT-REP string class? I think done properly, you'd see a large increase in performance across the board. There are many reasons why:

* Simpler data structures compared to R's heavy CHARSXP and R's global string cache
* [Short string optimization](https://blogs.msmvps.com/gdicanio/2016/11/17/the-small-string-optimization/)
* The possibility of true multithreading (you can't multithread R internals)

If there's interest, I'd be happy to develop and work on it.

To flesh it out a bit, I think you could use an ALT-REP class that's represented by standard STL structures:

```
std::vector
```

You don't need to keep track of encoding, if you can assume UTF-8.

You'd probably want some global configuration parameter:

```
stri_use_alt_rep(bool)
```

You'd have to replace every interaction with R string memory with a conditional.

```
CHAR
SET_STRING_ELT
STRING_ELT
mkChar*
Rf_allocVector(STRSXP,...)
```

And replace any comparison of address for testing string equality (not sure if stringi does so).

There are probably things I'm forgetting and it's a lot of work, but I think clearly defined.