Test fixes from rolling up PRs

Closes #12797 (Return Option rather than fail for `slice_shift_char` and {shift,pop}_{char,byte}) r=huonw
Closes #12803 (std: Relax an assertion in oneshot selection) r=brson
Closes #12818 (green: Fix a scheduler assertion on yielding) r=brson
Closes #12819 (doc: discuss try! in std::io) r=alexcrichton
Closes #12820 (Use generic impls for `Hash`) r=alexcrichton
Closes #12823 (rustuv: Fix a use-after-free in TTY failure) r=pcwalton
Closes #12826 (Remove remaining nolink usages) r=alexcrichton
Closes #12835 (Emacs: always jump the cursor if needed on indent) r=brson
Closes #12838 (Json method cleanup) r=alexcrichton
Closes #12843 (rustdoc: whitelist the headers that get a § on hover) r=alexcrichton
Closes #12844 (docs: add two unlisted libraries to the index page) r=pnkfelix
Closes #12845 (Vec: remove the drop flag to make it no larger than (*T, uint, uint).) r=pnkfelix
Closes #12846 (Added a test that checks that unary structs can be mutably borrowed) r=sfackler
Closes #12847 (mk: Fix warnings about duplicated rules) r=nmatsakis