doc: Reorganize the library index

Only single out std. Put everything else in a consistent list.
Ignore /build even if it’s a symlink.
auto merge of #12312 : brson/rust/doc, r=alexcrichton

Only single out std. Put everything else in a consistent list.
auto merge of #12319 : SimonSapin/rust/build-symlink, r=alexcrichton
Allow configuration of uid/gid/detach on processes

This just copies the libuv implementation for libnative which seems reasonable
enough (uid/gid fail on windows).

Closes #12082
Upgrade libuv

This notably includes joyent/libuv@6f62d62c in order to fix when processes fail
to spawn on windows
auto merge of #12085 : alexcrichton/rust/issue-12082, r=brson

This just copies the libuv implementation for libnative which seems reasonable
enough (uid/gid fail on windows).

Closes #12082
Remove Freeze bounds from sync::MutexArc

With Gc no longer trying to statically prevent cycles (and thus no
longer using the Freeze bound), it seems appropriate to remove that
restriction from MutexArc as well.