stri_encode: maximal supported size of a single string is ~0.67 GB

To support the full 2^31-1 (\*) bytes per string, the `stri_encode` would need to be rewritten using batch uconv processing.

(\*) [R Internals](https://cran.r-project.org/doc/manuals/r-release/R-ints.html):

> *Elements of character vectors (CHARSXPs) remain limited to 2^31 - 1 bytes.*

```r
> library(stringi); x<-stri_dup("a", 2**30); y <- paste(c(x, x), collapse="")
#Error in paste(c(x, x), collapse = "") : result would exceed 2^31-1 bytes
```