This commit changes the way we shell out externals when using the `"$it"` argument. Also pipes per row to an external's stdin if no `"$it"` argument is present for external commands.
Further separation of logic (preparing the external's command arguments, getting the data for piping, emitting values, spawning processes) will give us a better idea for lower level details regarding external commands until we can find the right abstractions for making them more generic and unify within the pipeline calling logic of Nu internal's and external's.
* Clippy fixes
* Finish converting to use clippy
* fix warnings in new master
* fix windows
* fix windows
Co-authored-by: Artem Vorotnikov <artem@vorotnikov.me>
Adds modules for internal, external, and dynamic commands, as well as
the pipeline functionality. These are exported as their old names from
the classified module so as to keep its "interface" the same.
