Feature request: equivalent of Beast's flat_buffer

[anarthal]

Protocols handling small messages can't use buffers like Asio's and Capy's dynamic_string_buffer, since these memmove on every consume. I'd like to see a buffer type in Capy like Beast's flat_buffer.

I personally don't need allocator customization, but I understand this could be useful to someone. Maybe using a PMR resource for allocation, rather than templates, could be beneficial?