* Remove completion code from help/value shells
* Tweak the `Completer` trait for nushell.
Previously, this trait was built around rustyline's completion traits, and for
`Shell` instances. Now it is built for individual completers inside of nushell
that will complete a specific location based on a partial string. For example,
for completing a partially typed command in the command position.
The completion engine maps completion locations to spans on a line, which
indicate whther to complete a command name, flag name, argument, and so on.
Initial implementation is simplistic, with some rough edges, since it relies
heavily on the parser's interpretation. For example
du -
if asking for completions, `-` is considered a positional argument by the
parser, but the user is likely looking for a flag. These scenarios will be
addressed in a series of progressive enhancements to the engine.
