Deprecate runtime usage parser

[epage]

The runtime usage parser adds extra code (complexity / compile time), defers errors to runtime, and the syntax is driven by the parser implementation, leading to some odd behavior.

We should replace it with a subset of the clap_app macro so users still get a usage parser but the cost is opt-in and errors are at compile time.

[epage]

Deprecated in 3c3a0b7

[epage]

If users want to fork this into its own crate to support it, some relevant commits

• 7604b1e
• 77b1adc
• 88fff13

[epage]

For an example of porting from Arg::from_usage to arg!, see 4c4a2b8