Associate default return types for different shell commands

[Jon Pretty]

A shell process may be defined as, for example, sh"echo foo" and is currently typed as Command,
but could be given a singleton type parameter as Command["echo"] since the command is known
statically.

A contextual typeclass instance could then associate the result of any "echo" command with the
return type, String (or a different type for other commands), such that the String in
exec[String]() could be inferred.

I think this requires changes to Contextual to allow the return type to be more specific.

[Jon Pretty]

This should be achievable by calling the Contextual macro from another macro, which refines the return type.