Add ability to provide named/typed args from a struct

This is the reverse of the usage of `argmapper.Struct` embedding.

I'd like to be able to let users pass a struct and then take all the exported fields of the struct and specify them as `Named` / `Typed` parameters (potentially reusing the same field tags as `argmapper.Struct` or some other configuration options).

```go
target, err := argmapper.NewFunc(func(w io.Writer) {
// ... use the writer ...
})

myStruct := &struct{
W io.Writer
}{ ... }

result := target.Call(argmapper.FromStruct(myStruct)) // FromStruct is the new method for the Arg
if result.Err() != nil {
panic(result.Err())
}
```

(written up from a conversation outside of GitHub)