rustc: Remove special treatment for Freeze and NoFreeze

Fixes #12577
arena: Remove Freeze / NoFreeze
sync: Remove Freeze / NoFreeze
std::comm: Remove Freeze / NoFreeze
cell: Remove Freeze / NoFreeze
std: Remove the Freeze kind and the NoFreeze marker
test: Remove Freeze / NoFreeze from tests
doc: Remove Freeze / NoFreeze from docs
Add Vec::mut_slice_from(), mut_slice_to(), and mut_split_at()
auto merge of #12995 : Kroisse/rust/vec_ng_mut_slices, r=alexcrichton

These functions are implemented for &mut [T], but std::vec_ng::Vec has not yet.
Get rid of @Cell
Unindent stringifier() in tasks guide
auto merge of #13076 : FlaPer87/rust/remove-freeze, r=alexcrichton

This PR removes the `Freeze` kind and the `NoFreeze` marker completely.

Fixes #12577

cc @nikomatsakis r?
auto merge of #13084 : Havvy/rust/master, r=alexcrichton

The indentation looks off in the guide because the main() function is not shown, so I'm dedenting the visible function.
Implement vtable support for autoderef.
Closes #13042.
auto merge of #13085 : edwardw/rust/saddest-cell, r=cmr
Implement cross-crate support for autoderef.
Closes #13044.
auto merge of #13087 : eddyb/rust/fix-autoderef, r=cmr

Implements vtable support for generic Deref impls with trait bounds.
Also fixes cross-crate inlining when using autoderef.
make std::managed private

This removes two tests built on `managed::refcount`, but these issues
are well-covered elsewhere for non-managed types.
auto merge of #13089 : thestinger/rust/managed, r=Aatch

This removes two tests built on `managed::refcount`, but these issues
are well-covered elsewhere for non-managed types.