std: Add cfg(test) to UnsafeArc assertions

This is a ubiquitous type in concurrent code, and the assertions are causing
significant code bloat for simple operations such as reading the pointer
(injecting a failure point, etc).

I am testing executable sizes with no I/O implementations (everything stubbed
out to return nothing), and this took the size of a libnative executable from
328K to 207K (37% reduction in size), so I think that this is one assertion
that's well worth configuring off for now.
std: Remove lots of allocations from log settings

Most of these are unnecessary because we're only looking at static strings. This
also moves to Vec in a few places instead of ~[T].

This didn't end up getting much of a code size win (update_log_settings is the
third largest function in the executables I'm looking at), but this seems like a
generally nice improvement regardless.
std: Avoid using "{:?}" in format strings

This removes all usage of Poly in format strings from libstd. This doesn't
prevent more future strings from coming in, but it at least removes the ones for
now.
std: Flag run_fmt() as #[inline(always)]

This function is a tiny wrapper that LLVM doesn't want to inline, and it ends up
causing more bloat than necessary. The bloat is pretty small, but it's a win of
at least 7k for small executables, and I imagine that the number goes up as
there are more calls to fail!().