Documentation and adjustments to `clear'-related hash and set operations

Document and adjust `hash-clear!', `hash-clear', and `hash-empty?'.

Also, add `hash-copy-clear'.

The clear operations are constant-time for a non-impersonated
hash table, otherwise they always remove keys one-by-one to
trigger the impersonator's interpositions.

The `hash-clear' operation works only on immutable hash tables,
in contrast to the original implementation. The new `hash-copy-clear'
works on both immutable and mutable hash tables. The "copy"
in its name is meant to suggest a difference with `hash-clear',
even on immutable hash tables: any chaperone on the input
is not on the outpue.

Change `set-clear' to be like `hash-clear', and add
`set-copy-clear'.

(Changes are in consultation with Carl.)
raco pkg update: allow packages sources as replacements

When an argument to `raco pkg update` is a package source,
use it to place the currently installed package.

Also, make the set of available command-line arguments more
consistent, especially for `raco pkg install` and `raco pkg update`.

Finally, fix the `--update-deps` flag, including checking
the dependencies of each updated packages based on then update,
instead of the pre-updated package.