allow trailing slash in standard-library redirects

[jyn514]

fixes #474

[emilyalbini]

I'd prefer not to duplicate the list of crates we redirect. A solution could be to create a static like this:

static DOC_RUST_LANG_ORG_REDIRECTS: &[&str] = &["alloc", "std", ...];

...and dynamically generate the routes?

[jyn514]

@pietroalbini I removed the duplicate code, want to take another look?

[emilyalbini]

Looks great, thanks!