Reduce allocations when printing commands (#20)

Removed the usage of `std::map` and a large bunch of string allocations when doing auto completion.

A side effect of not using the map; the commands are no longer sorted! It's now the consumer who has to provide the order.
Could add a constexpr sorting helper to operate on the span but it will require that we remove some constness.

Also did some renaming to make it more clear about what's going on.

Author: hansbinderup