String manipulation functions added

[NemesisWasAlienToo]

I have added some basic functionality for compile-time manipulation such as find, count, split and replace.
If this poll request is accepted I will try to add compile-time regex::replace too.

[hanickadot]

What's the motivation for this change? Only usage for ctll::fixed_string is ability to store pattern in order to parse it. Also you can always just cast it into std::u32string_view and do the operations there.

[NemesisWasAlienToo]

Im am working on my web framework and in it am using ctre for routing.
I needed these utilitties in order to create the right compile time regex from user input and to later on use it inside my router.

[NemesisWasAlienToo]

What should i do now to get it to merge?

[hanickadot]

Explain me why it needs to be part of CTRE and not your library. Seems like your functions are pretty standalone. What's the additional value for CTRE?

[NemesisWasAlienToo]

It can be used to implement ctre::repace functionality and do compile time regex replacement.
And besides I think it can be useful to have compile time string manipulation facilities at hand.
Since other libraries might try to use ctre and they need to implememt these functions every time and later find a way to construct ctll::fixed_string from their own version of it.
So i can be beneficial to have them at hand.

[hanickadot]

It's a lot of maybe. Nothing you are currently trying to add is needed for CTRE. I would prefer to merge or see complete replace implementation if you want so. Then I can decide what's needed and in which form.

[NemesisWasAlienToo]

Fair enough

[brouhaha]

I desperately needed the ability to concatenate a ctll::fixed_string template parameter with a string literal in the template, and found this very useful as I didn't previously have any idea how to do that myself. I'm not trying to disagree with the idea that this doesn't need to be in the ctre package, but if it hadn't been in this pull request, I probably would have been pestering someone for assistance.
I used ideas from this together with an idea from https://stackoverflow.com/a/62823211 to get a slightly different variant.