Refactor writeback code. cc #5527
Remove error message that refactor suppresses
Add testcase for #11493
std: Extract format string parsing out of libstd

This code does not belong in libstd, and rather belongs in a dedicated crate. In
the future, the syntax::ext::format module should move to the fmt_macros crate
(hence the name of the crate), but for now the fmt_macros crate will only
contain the format string parser.

The entire fmt_macros crate is marked #[experimental] because it is not meant
for general consumption, only the format!() interface is officially supported,
not the internals.

This is a breaking change for anyone using the internals of std::fmt::parse.
Some of the flags have moved to std::fmt::rt, while the actual parsing support
has all moved to the fmt_macros library.

[breaking-change]
auto merge of #13985 : alexcrichton/rust/libfmt, r=brson

This code does not belong in libstd, and rather belongs in a dedicated crate. In
the future, the syntax::ext::format module should move to the fmt_macros crate
(hence the name of the crate), but for now the fmt_macros crate will only
contain the format string parser.

The entire fmt_macros crate is marked #[experimental] because it is not meant
for general consumption, only the format!() interface is officially supported,
not the internals.

This is a breaking change for anyone using the internals of std::fmt::parse.
Some of the flags have moved to std::fmt::rt, while the actual parsing support
has all moved to the fmt_macros library.

[breaking-change]
auto merge of #13990 : nikomatsakis/rust/issue-5527-cleanup-writeback, r=pcwalton

As part of #5527 I had to make some changes here and I just couldn't take it anymore. Refactor the writeback code. Should be functionally equivalent to the old stuff.

r? @pcwalton
auto merge of #14010 : richo/rust/tests/11493, r=alexcrichton

Is this what you had in mind?

Closes #11493
add back jemalloc to the tree

This adds a `std::rt::heap` module with a nice allocator API. It's a
step towards fixing #13094 and is a starting point for working on a
generic allocator trait.

The revision used for the jemalloc submodule is the stable 3.6.0 release.

Closes #11807