`echo` tends to confuse new Nu users; they expect it to work like
`print` when it just passes a value to the next stage of the pipeline.
We haven't quite figured out what to do about `echo` in the long run,
but I think a good start is to remove `echo` from command examples where
it would be unnecessary and arguably unidiomatic.
* Add failing test that list of ints and floats is List<Number>
* Start defining subtype relation
* Make it possible to declare input and output types for commands
- Enforce them in tests
* Declare input and output types of commands
* Add formatted signatures to `help commands` table
* Revert SyntaxShape::Table -> Type::Table change
* Revert unnecessary derive(Hash) on SyntaxShape
Co-authored-by: JT <547158+jntrnr@users.noreply.github.com>
