syntax: allow `trace_macros!` and `log_syntax!` in item position.

Previously

    trace_macros!(true)
    fn main() {}

would complain about `trace_macros` being an expression macro in item
position. This is a pointless limitation, because the macro is purely
compile-time, with no runtime effect. (And similarly for log_syntax.)

This also changes the behaviour of `trace_macros!` very slightly, it
used to be equivalent to

    macro_rules! trace_macros {
        (true $($_x: tt)*) => { true };
        (false $($_x: tt)*) => { false }
    }

I.e. you could invoke it with arbitrary trailing arguments, which were
ignored. It is changed to accept only exactly `true` or `false` (with no
trailing arguments) and expands to `()`.
native: Fix a possible deadlock in spawn

The OSX bots have been deadlocking recently in the rustdoc tests. I have only
been able to rarely reproduce the deadlock on my local setup. When reproduced,
it looks like the child process is spinning on the malloc mutex, which I
presume is locked with no other threads to unlock it.

I'm not convinced that this is what's happening, because OSX should protect
against this with pthread_atfork by default. Regardless, running as little code
as possible in the child after fork() is normally a good idea anyway, so this
commit moves all allocation to the parent process to run before the child
executes.

After running 6k iterations of rustdoc tests, this deadlocked twice before, and
after 20k iterations afterwards, it never deadlocked. I draw the conclusion that
this is either sweeping the bug under the rug, or it did indeed fix the
underlying problem.
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
std::os: Handle FormatMessage failure

`FormatMessageW()` is called by `std::os::last_os_error()` to convert
errno into string, but the function may fail on non-english locale.
I don't know why it fails, but anyway it's better to return errno
than to `fail!()` in the case.

Fixes #13075
Fixes #13073
auto merge of #13078 : klutzy/rust/issue-13075, r=alexcrichton

`FormatMessageW()` is called by `std::os::last_os_error()` to convert
errno into string, but the function may fail on non-english locale.
I don't know why it fails, but anyway it's better to return errno
than to `fail!()` in the case.

Fixes #13075
Fixes #13073
std: Remove the get() method from RefCell wrappers

This method has been entirely obsoleted by autoderef, so there's no reason for
its existence.
syntax: Fix fallout of removing get()
arena: Fix fallout of removing get()
rustc: Fix fallout of removing get()
rustdoc: Fix fallout of removing get()
test: Fix fallout of removing get()
rustc: Remove all usage of manual deref()

Favor using '*' instead
rustuv: Remove usage of get()