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]