string: add length management functions

[freshFruict]

• cista::generic_string: add internal_change_capacity method
• cista::string: add resize, reserve and shrink_to_fit methods

[felixguendling]

The idea of self_allocated_ was previously that the string knows if it needs to free or not - especially if you deserialize a buffer and modify the string. In this case it might allocate and will then switch from self_allocated=false to self_allocated=true. Will this still work?

[freshFruict]

It's supposed to behave the same way. The idea is that string stores the capacity of the buffer it owns, and the capacity of non-owning string is set to 0, so calling any size modification method will result in the original string being copied, with memory allocation if required

Although looking now, I see I've misused h.reserved_ on lines 221 and 224, it won't work properly on BE systems. I'll fix that, and probably refactor the new code slightly

[freshFruict]

Now I think I've got nothing to add to this PR

[felixguendling]

Does this mean that the allocation will be erased if the string has been allocated (ie. was longer than short length limit at one point)?

I am thinking about use cases similar to this one where you want that it allocates a few times until it has a size that will never require new allocations anymore.

std::string s;
while (std::getline(s)) { /* ... */ }

[freshFruict]

I see, that is likely not the best decision. I will look into it