Use compact valueName by default

By default, `ArgumentParser` currently uses a `ArgumentHelp.valueName` derived from the option name. While this is a nice standard default, I think it often leads to overly verbose --help text, and customizing with `ArgumentHelp` makes declaration a little verbose (super kudos for having it possible in v0.0.1 though!).

I think there is an argument to be made for having `ArgumentParser` use a default value name for well-known and common types, for example, using `N` for integral types.

One upside of this is that it helps communicate the expected type.

I can understand if this is perceived to be too much magic, though...

Start by reading ArgumentParser's handling of ArgumentHelp.valueName and the default value name derived from option names. Determine which well-known types should receive compact names such as N, then verify the resulting --help output and settle how the defaults should be configured.

Written by the indexing model from the issue text.