Provisional implementation of object appending via bind/new

In Brian Hawley's proposal, appending to an `object!` employs three
passes:

1. Verify the `block!` argument
2. Add any absent words to the target `object!` with `bind/new`
3. Update values for each word, favouring the latest, using `set/any`

In this commit, (2) and (3) are merged.