Make `Descriptor::new` `const`

[mxinden]

Consider making Descriptor::new const. This would be handy when used with Collector when passing a borrowed 'static `Descriptor.

As far as I can tell, the only current blocker is the string manipulation to add a ,:

client_rust/src/registry.rs

Lines 437 to 438 in 85033bf

	let help = help.into() + ".";

[mxinden]

With #149 this is obsolete.