std: reduce the generic code instantiated by fail!().

This splits the vast majority of the code path taken by
`fail!()` (`begin_unwind`) into a separate non-generic inline(never)
function, so that uses of `fail!()` only monomorphise a small amount of
code, reducing code bloat and making very small crates compile faster.
std: add begin_unwind_fmt that reduces codesize for formatted fail!().

This ends up saving a single `call` instruction in the optimised code,
but saves a few hundred lines of non-optimised IR for `fn main() {
fail!("foo {}", "bar"); }` (comparing against the minimal generic
baseline from the parent commit).
auto merge of #11841 : huonw/rust/noinline-fail, r=alexcrichton

In two ways:
- for a plain `fail!(a)` we make the generic part of `begin_unwind` as small as possible (makes `fn main() { fail!() }` compile 2-3x faster, due to less monomorphisation bloat)
- for `fail!("format {}", "string")`, we avoid touching the generics completely by doing the formatting in a specialised function, which (with optimisations) saves a function call at the call-site of `fail!`. (This one has significantly less benefit than the first.)