Support for `resize` [perhaps also `capacity` and `reserve`] for cista::generic_string/basic_string

[ChemistAion]

There is a usage pattern for string like types when internal buffer (with known size a priori)] is filled later, e.g.: https://github.com/ocornut/imgui/blob/master/misc/cpp/imgui_stdlib.cpp

Could you please consider to add resize() or even more - additionally with: capacity() and reserve() to mimic what the gang from std does... just to handle such callback-cases in a elegant way :innocent: