Const-ify the descriptor on Message

[cyang1]

This makes it possible to use the descriptor from a const context.

[goffrie]

For some reason I had thought you wanted this to be &self for object-safety, but of course Message already isn't object-safe so that doesn't make sense. This seems better.

[cyang1]

Got back around to updating this.

I think because the clippy CI command is cd-ing into the proto_pbtest directory and then running cargo clippy, it's not picking up the [patch.crates-io] from pb-test/Cargo.yaml. Not sure what we want to do about that.

[goffrie]

this causes a double space :p
though is there any point to having the parens at all?

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