Prototype out a help trait

[erickt]

This is a prototype to explore an alternative to #115. The idea here is that we change argh_derive to produce an implementation of a new Help trait, that has an associated constant that describes the trait. I've changed argh_derive to use this helper to produce the help string.

There's a couple things I'm not crazy about:

• It produces larger binaries. I'm guessing because the help strings have a bunch of extra machinery to construct the strings. However the size might be smaller if you use the Help trait at the same time as FromArgs traits.
• I did a bunch of copy-paste, so there's a bit of duplication going on.
• argh::CommandInfo is made somewhat, but not completely, redundant. We could merge the two together at the cost of a breaking api change, but that might be worth it.
• I didn't investigate, but we might be able to also support the use cases in Add support for an omit_usage attribute #75, Manually print usage #56, Expose HelpMessage and allow for custom help switches #53, Allow positional arguments to be omitted from the help text #73, and etc.
• I'm not crazy with how I had to handle examples and notes, but the old help allowed for command_name substitution, so I used a closure to prepare those strings.

cc @claywilkinson and @richkadel

[erickt]

Pushed up some new patches to:

• Move the help traits into argh_shared, and use some of them in argh_derive. Unfortunately I don't have a great way to deal with subcommand in argh_derive though.
• Got rid of using curried format functions for examples and notes, and replaced with a simple string replacement. It's not quite a correct substitution though, since it'll replace {{command_name}}, but the old argh_derive will not.
• Added an optional serde implementations.