* 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>
Now, Echo converts multiple values in a ValueStream, but it simply
forwards a single Value; if no PipelineData is detected as an input, an
empty string is returned as a single Value.
Values to echo need to be extracted from the call, and then converted
into PipelineData.
I also updated the first example so that its result is a List,
as in the reference implementation.
