rust: add "bits", a custom bitflags implementation

One common thing that device emulation does is manipulate bitmasks, for example
to check whether two bitmaps have common bits. One example in the

rust: add "bits", a custom bitflags implementation

One common thing that device emulation does is manipulate bitmasks, for example
to check whether two bitmaps have common bits. One example in the pl011 crate
is the checks for pending interrupts, where an interrupt cause corresponds to
at least one interrupt source from a fixed set.

Unfortunately, this is one case where Rust *can* provide some kind of
abstraction but it does so with a rather Perl-ish There Is More Way To
Do It. It is not something where a crate like "bilge" helps, because
it only covers the packing of bits in a structure; operations like "are
all bits of Y set in X" almost never make sense for bit-packed structs;
you need something else, there are several crates that do it and of course
we're going to roll our own.

In particular I examined three:

- bitmask (https://docs.rs/bitmask/0.5.0/bitmask/) does not support const
at all. This is a showstopper because one of the ugly things in the
current pl011 code is the ugliness of code that defines interrupt masks
at compile time:

pub const E: Self = Self(Self::OE.0 | Self::BE.0 | Self::PE.0 | Self::FE.0);

or even worse:

const IRQMASK: [u32; 6] = [
Interrupt::E.0 | Interrupt::MS.0 | Interrupt::RT.0 | Interrupt::TX.0 | Interrupt::RX.0,
...
}

You would have to use roughly the same code---"bitmask" only helps with
defining the struct.

- bitmask_enum (https://docs.rs/bitmask-enum/2.2.5/bitmask_enum/) does not
have a good separation of "valid" and "invalid" bits, so for example "!x"
will invert all 16 bits if you choose u16 as the representation -- even if
you only defined 10 bits. This makes it easier to introduce subtle bugs
in comparisons.

- bitflags (https://docs.rs/bitflags/2.6.0/bitflags/) is generally the most
used such crate and is the one that I took most inspiration from with
respect to the syntax. It's a pretty sophisticated implementation,
with a lot of bells and whistles such as an implementation of "Iter"
that returns the bits one at a time.

The main thing that all of them lack, however, is a way to simplify the
ugly definitions like the above. "bitflags" includes const methods that
perform AND/OR/XOR of masks (these are necessary because Rust operator
overloading does not support const yet, and therefore overloaded operators
cannot be used in the definition of a "static" variable), but they become
even more verbose and unmanageable, like

Interrupt::E.union(Interrupt::MS).union(Interrupt::RT).union(Interrupt::TX).union(Interrupt::RX)

This was the main reason to create "bits", which allows something like

bits!(Interrupt: E | MS | RT | TX | RX)

and expands it 1) add "Interrupt::" in front of all identifiers 2) convert
operators to the wordy const functions like "union". It supports boolean
operators "&", "|", "^", "!" and parentheses, with a relatively simple
recursive descent parser that's implemented in qemu_api_macros.

Since I don't remember exactly how the macro was developed, I cannot exclude
that it contains code from "bitflags". Therefore, I am conservatively leaving
in the MIT and Apache 2.0 licenses from bitflags. In fact, I think there
would be a benefit in being able to push code back to "bitflags" anyway
whenever applicable, so that the two libraries do not diverge too much,
so that's another reason to use this.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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