rustdoc: Hyperlink cross-crate reexports

This should improve the libcore experience quite a bit when looking at the
libstd documentation.
doc: Fix some broken links
Register new snapshots
auto merge of #14068 : alexcrichton/rust/rustdoc-xcrate-links, r=brson

This should improve the libcore experience quite a bit when looking at the
libstd documentation.
auto merge of #14073 : alexcrichton/rust/snapshots, r=huonw