Move mutable::Mut to cell::RefCell
extra: improve the errors for the JSON Decoder.

Fixes #4244.
auto merge of #10514 : sfackler/rust/mut, r=cmr

This is based off of @blake2-ppc's work on #9429. That PR bitrotted and I haven't been able to contact the original author so I decided to take up the cause.

Overview
======
`Mut` encapsulates a mutable, non-nullable slot. The `Cell` type is currently used to do this, but `Cell` is much more commonly used as a workaround for the inability to move values into non-once functions. `Mut` provides a more robust API.

`Mut` duplicates the semantics of borrowed pointers with enforcement at runtime instead of compile time.
```rust
let x = Mut::new(0);

{
    // make some immutable borrows
    let p = x.borrow();
    let y = *p.get() + 10;

    // multiple immutable borrows are allowed simultaneously
    let p2 = x.borrow();

    // this would throw a runtime failure
    // let p_mut = x.borrow_mut();
}

// now we can mutably borrow
let p = x.borrow_mut();
*p.get() = 10;
```
`borrow` returns a `Ref` type and `borrow_mut` returns a `RefMut` type, both of which are simple smart pointer types with a single method, `get`, which returns a reference to the wrapped data.

This also allows `RcMut<T>` to be deleted, as it can be replaced with `Rc<Mut<T>>`.

Changes
======
I've done things a little bit differently than the original proposal.

* I've added `try_borrow` and `try_borrow_mut` methods that return `Option<Ref<T>>` and `Option<RefMut<T>>` respectively instead of failing on a borrow check failure. I'm not totally sure when that'd be useful, but I don't see any reason to not put them in and @cmr requested them.
* `ReadPtr` and `WritePtr` have been renamed to `Ref` and `RefMut` respectively, as `Ref` is to `ref foo` and `RefMut` is to `ref mut foo` as `Mut` is to `mut foo`.
* `get` on `MutRef` now takes `&self` instead of `&mut self` for consistency with `&mut`. As @alexcrichton pointed, out this violates soundness by allowing aliasing `&mut` references.
* `Cell` is being left as is. It solves a different problem than `Mut` is designed to solve.
* There are no longer methods implemented for `Mut<Option<T>>`. Since `Cell` isn't going away, there's less of a need for these, and I didn't feel like they provided a huge benefit, especially as that kind of `impl` is very uncommon in the standard library.

Open Questions
============
* `Cell` should now be used exclusively for movement into closures. Should this be enforced by reducing its API to `new` and `take`? It seems like this use case will be completely going away once the transition to `proc` and co. finishes.
* Should there be `try_map` and `try_map_mut` methods along with `map` and `map_mut`?
auto merge of #10578 : luqmana/rust/mingw64, r=alexcrichton

With these changes I was able to cross compile for windows from a linux box. (Using the mingw-w64 package on Debian Testing).

Fixed a bug where the `target_family` cfg would be wrong when targeting something with a different value than the host. (i.e windows -> unix or unix -> windows).

Also, removed `LIBUV_FLAGS` in `mk/rt.mk` because of the redundancy between it and `CFG_GCCISH_CFLAGS_(target)`.

After this we can create a snapshot and migrate to mingw64 instead of mingw32.
auto merge of #10620 : cmr/rust/linker_opts, r=thestinger
auto merge of #10625 : huonw/rust/json-errors, r=alexcrichton

Fixes #4244.
std::libc: Remove TCHAR types
std::libc: Simplify win32/win64 type definitions
std: Fix transmute error on win64
llvm: Disable pthreads on mingw-w64 platforms
std::rt: Fix record_stack_bounds on win64
std::rt: Fix crate_map on Win64
Use -O1 for non-GNU linkers
auto merge of #10633 : cmr/rust/linker_opts, r=thestinger

r? @Luqmana
Implement cmp traits for Rc<T> and add a ptr_eq method.
auto merge of #10634 : LeoTestard/rust/rc-eq, r=cmr
Fix issue #8763

* added case & better error message for "impl trait for module"
* used better way to print the module
* switched from //error-pattern to //~ ERROR
* added compile-fail test trait-impl-for-module.rs
* revised compile-fail test trait-or-new-type-instead
    (the error message for the modified test is still unclear, but that's a different bug)
* added FIXME to trait-or-new-type-instead
auto merge of #10475 : astrieanna/rust/issue8763, r=alexcrichton

Issue #8763 is about improving a particular error message.

* added case & better error message for "impl trait for module"
* added compile-fail test trait-impl-for-module.rs
* updated copyright dates
* revised compile-fail test trait-or-new-type-instead
   (the error message for the modified test is still unclear, but that's a different bug https://github.com/mozilla/rust/issues/8767)
std::trie: Fix find_mut for non-present keys

Make TrieMap/TrieSet's find_mut check the key for external nodes.
Without this find_mut sometimes returns a reference to another key when
querying for a non-present key.
libextra: Remove @mut from term.