rustdoc should preserve case in anchors

[metajack]

Right now a function named FILE_writer and file_writer will both get lowercased in their anchor tags, causing the latter to point to the former, or vice verse depending on the ordering.

Rust identifiers are case-sensitive, so anchors should be too.

[pnkfelix]

visiting for triage, email 2013-08-12.

@cmr

Probably part of #8125

[emberian]

cc @Seldaek; if/when this feature is added, this is a consideration.

[alexcrichton]

This is done as much as it's going to get done. Right now we do have anchors on trait methods which obey this case-preservation, and I think that we should just keep this in mind for the future when we add more anchors.