maybe implement command invocation-by function-ref

[ichthyo]

There is a variant of the invoke(..) (free function), which identifies the command to be invoked not by a symbolic ID, but directly by a function-ref. Doing so, would yield two benefits:

• the argument number and types can be checked statically, even when only provided at invocation time
• it would allow to make a command invocation look like a direct invocation of the function, thereby hiding the presence of the command framework altogether

I haven't fully implemented this feature though, the unimplemented hook is within CommandRegistry. It would require to maintain an additional index, and moreover solve the (tricky) problem of functor identity and comparisons. It depends laregely on the actual usage situation of the command framework, if such a feature is worth the effort.