feat: change bindcom.Command to a struct

[alanshaw]

re: fil-forge/libforge#26 (comment)

Instead of:

// content/retrieve.go

Retrieve := bindcom.Parse[*RetrieveArgs]("/content/retrieve")

// server.go

s := retrieval.NewServer(service)
s.Handle(ucan.Command(content.Retrieve), /* ... */)

You can do:

s.Handle(content.Retrieve.Command, /* ... */)

Alternative to #15

[Peeja]

Yeah, this seems like it works well enough. 👍🏻