rework `swank:simple-completions`

In the process, implement looking up foo$bar$baz, and passing those
completions back.  It's not completely robust to somewhat exotic
syntax, as it assumes that the text being completed can be used
directly as character vectors naming objects or fields; it is good
enough to get started, and now a lot less annoying to
use (particularly when lots of fields have underscores in them...)