Refine privacy error messages to be more accurate

This stops labeling everything as "is private" when in fact the destination may
be public. Instead, the clause "is inaccessible" is used and the private part of
the flag is called out with a "is private" message.

Closes #9793
Document traits and Default about format! better

Closes #9865
Closes #9808
Remove the fmt! syntax extension

It lived a good life, but its time has come. The groundwork is set for the
official transition after the next snapshot (removal of XXX2 macros)
auto merge of #9856 : alexcrichton/rust/privacy-errors, r=huonw

This stops labeling everything as "is private" when in fact the destination may
be public. Instead, the clause "is inaccessible" is used and the private part of
the flag is called out with a "is private" message.

Closes #9793
auto merge of #9919 : alexcrichton/rust/fmt-begone, r=huonw

It lived a good life, but its time has come. The groundwork is set for the
official transition after the next snapshot (removal of XXX2 macros)
rustpkg: Make rustpkg tests stop comparing dates

Instead of scrutinizing modification times in rustpkg tests,
change output files to be read-only and detect attempts to write
to them (hack suggested by Jack). This avoids time granularity problems.

As part of this change, I discovered that some dependencies weren't
getting written correctly (involving built executables and library
files), so this patch fixes that too.

This partly addresses #9441, but one test (test_rebuild_when_needed)
is still ignored on Linux.
rustc: Check that the output file is writeable before linking

This is because on Linux, the linker will silently overwrite
a read-only file.
rustpkg: invoke touch with a portable set of args
auto merge of #9437 : catamorphism/rust/rustpkg-dates, r=alexcrichton

r? @alexcrichton On most platforms, the time granularity is 1 sec or more, so comparing
dates in tests that check whether rebuilding did or didn't happen leads
to spurious failures. Instead, test the same thing by making an output
file read-only and trapping attempts to write to it.
auto merge of #9938 : alexcrichton/rust/fmtdox, r=huonw

Closes #9865
Closes #9808
Set external linkage on Rust functions defined with foreign ABIs.
test for extern C global linkage