Allow dependency to forcibly override profile settings

[DanielKeep]

Currently, I'm using lalrpop as part of a project. Because of the mammoth compile times involved, I've contained the parser in its own crate.

The problem is that not only is the parser very slow to compile, it is catastrophically slow in debug builds. As in, a release build runs around 20x faster. As such, I want my parser package to always be compiled with optimisations enabled to keep from going completely insane.

But, I can't. I want to be able to have a debug build that isn't heavily optimised (save for this one dependency). A way to specify that a package's profile settings take precedence would be great, although I'd also settle for a way to specify the profile settings for particular dependencies in the top-level package's manifest, too.

Please. It's taking eight seconds to parse a few KBs worth of text. It's ridiculous.

[alexcrichton]

Thanks for the report! There's been some discussion over on #2424, #1359, and this is kinda a dupe of those so I'm going to close in favor of them.