Replace instances of stringr::str_sub() with base R equivalent

[rich-iannone]

This replaces all instances of stringr::str_sub() with the base R equivalent str_substitute().

[yihui]

Perhaps you borrowed str_substitute() from your own packages, but we don't really need a sophisticated function here. The extract function stringr::str_sub() can be directly replaced by base::substr(), and the replacement function str_sub<- is a little trickier, since substr<- is not vectorized and has other quirks. Replacing parts of a string doesn't seem to be a complicated task, so I wrote the str_replace() function in utils-string.R, which seems to work (has passed the tests).

Thanks!

[rich-iannone]

Wow, thank you for taking this in another direction and right to the finish line! I was going to make the str_substitute<- function this weekend but now I don't have to. Alright! Onto the next one.