add from_bits_[ref|mut] for FFI in place ops

[flier]

When use FFI struct, we always need to copy the field to a flags and write it back after change it

#[repr(C)]
struct Foo {
   bar: u32,
}

let mut flags = Flags::from_bits_truncate(foo.bar);
// ...
flags |= Flags::SOME;
// ...
foo.bar = flags.bits();

With from_bits_ref and from_bits_mut method, we could support in place operations, like

let flags = Flags::from_bits_mut(&mut foo.bar);
// ...
*flags |= Flags::SOME;

Every change to the underlying field could be read immediately,
and the mutable reference can be pass/return to other function, no more write back.

[KodrAus]

Thanks for the PR @flier!

I'm a little concerned about introducing unsafe into the macros, because it will break folks compiling with #[deny(unsafe)], and leave no option for folks compiling with #[forbid(unsafe)].

The #[repr(transparent)] change looks like a good one though.

[flier]

I think we may introduce an unsafe feature for those scenes which need work with unsafe FFI code, and disable it by default for compatibility. @KodrAus

[KodrAus]

@flier Hmm, it would be unfortunate to leak the fact that this requires unsafe to implement that way. It's not a discoverable way to flag it. So I think our best path forward in bitflags is to add just the #[repr(transparent)] attribute so it's valid for you to write the cast in your own FFI code.

We could add a section under our crate docs called Layout or FFI, and note that your bitflags has the same size and alignment of T, but not all valid bit patterns of T are guaranteed to be valid flags.

[flier]

Fair, I have submit another PR #187 to add the #[repr(transparent)] attribute, thank anyway