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.
Reexport wchar_t, c_schar
auto merge of #14073 : alexcrichton/rust/snapshots, r=huonw
auto merge of #14081 : Manishearth/rust/c-schar-reexport, r=huonw

Don't merge this until #14076 merges, since it is based on top of that branch to avoid merge conflicts.