Inferring more meaning from types for derives

[epage]

Discussed in #2637

^{Originally posted by epage July 28, 2021}
Right now, we only use types to infer occurrences / values (see #1772) or some limited capabilities when a user sets arg_enum.

What else can we infer from types?

• If its a Path, set ValueHint::AnyPath
• Expand on fix(derive): Follow value_name convention #2611 and set value_name based on the type, like NUM for integers, PATH for Path, etc
• Select an appropriate parser for Path, OsString, etc.
  • feat: Auto-detect FromStr, TryFrom<&OsStr>, etc., in clap_derive #2298 wuld help with this

[epage]

This was greatly improved with the introduction of value_parser, see #3742