* Add input and output types to $nu.scope.commands
This commit changes the schema: instead of
command.signature: table
we now have
command.signatures: list<table>
with one signature for every input-output type pair.
* Represent signatures as a map from input_type to signature
* Sort signature entries
* Drop command name from signature tables
* Don't use "rest" as name of rest parameter; use empty string instead
* Bug fix: was creating records with repeated keys
E.g.
$nu.scope.commands | where name == 'hash sha256' | get signatures.0 | table -e
$nu.scope.commands | where name == 'transpose' | get signatures.0 | table -e
