Improve performance for long strings by preallocating the needed memory

[iscgar]

Instead of causing a regrow of the String's underlying buffer by the push loop and then by pushing s, use with_capacity to preallocate the memory.

[iscgar]

Oops. When I forked the repository #6 didn't exist, so I created this PR.
Sorry. Choose whichever suits you best (though I should note that my version was consistently a few ns faster in my benchmarks).

[iscgar]

This is actually silly, because len() returns the amount of bytes the str occupies, where people expect it to work on Unicode codepoints (.chars().chount() does that, but with UTF-8 it takes O(n) time). Rust is doing the right thing here, because the "length" of a string doesn't really mean anything. In fact, when people talk about string length they actually mean the amount of grapheme clusters, which most string libraries don't provide (the unicode-segmentation crate provides that functionality, but it's even slower).

Just an example of how even a simple thing like leftpad is incorrectly defined 😆

[hfiguiere]

leftpad-rs was written as a joke. So hasty that the first implementation didn't even match what I was trying to clone.