rustdoc: provide anchors for primitive types

[aturon]

rustdoc should have a landing page for primitive types.

Currently, there is nowhere to look in rustdoc for the list of methods that apply to &str, u8, or other primitive types. The std::str module is especially problematic, since it provides a Str trait with the sole as_slice method, which is very confusing to newcomers.

See #11409 for an earlier attempt to address this problem.

#6065 and #11526 may also be related.

I think this needs to happen before 1.0. Nominating.

[aturon]

cc @bjz

[brson]

1.0 P-low