* 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
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).
