Rust: Assume in the CFG that lowercase identifiers are in fact identifiers

[paldepind]

No description provided.

[paldepind]

DCA shows some changes in the number of CFG inconsistencies. Some thoughts on that:

• There are no changes for projects with 0 or close to 0 existing CFG inconsistencies. This is reassuring, since projects going from 0 inconsistencies to >0 inconsistencies would have been concerning.
• For projects that already have CFG inconsistencies, most now have fewer inconsistencies. I don't think this means anything. This PR shouldn't fix inconsistencies, but creating fewer edges probably just means whatever is wrong at those places isn't being hit anymore. Looking at the changes on coreutils matches that interpretation.
• We have two additional inconsistencies for databend. I looked at those and they're not because the heuristic we use is wrong. So while it does look like something we should investigate, it seems like an existing issue that is now revealed after this change.

In conclusion, I think merging this is fine :)

[redsun82]

LGTM, seems sensible