* Making Commands match what UntaggedValue needs
* WIP
* WIP
* WIP
* Moved to expressions for conditions
* Add 'each' command to use command blocks
* More cleanup
* Add test for 'each'
* Instead use an expression block
Previously, if the user didn't have the appropriate permissions to execute the
binary/script, they would see "command not found", which is confusing.
This commit eliminates the `which` crate in favour of `ichwh`, which deals
better with permissions by not dealing with them at all! This is closer to the
behaviour of `which` in many shells. Permission checks are then left up to the
caller to deal with.
This improves incremental build time when working on what was previously
the root package. For example, previously all plugins would be rebuilt
with a change to `src/commands/classified/external.rs`, but now only
`nu-cli` will have to be rebuilt (and anything that depends on it).
