static av1_intra_prediction_edges: Make non-mut by replacing #[derive(BitfieldStruct)] with bools_bitfield_struct! for const fns

[kkysen]

#[derive(c2rust_bitfields::BitfieldStruct)] doesn't generate const fns (immunant/c2rust#1027), so it can't be used in a const block to initialize a static. Thus, the static has to made mut and unsafe and initialized at runtime.

I opened immunant/c2rust#1027 and looked into making the generated fns const, but they used a lot of traits, which can't contain const fns, so it seemed like non-trivial work. I also tried looking for other off-the-shelf bitfield crates, but they all either didn't support const fns or still relied on traits within them, and thus only could create const fns with nightly features.

Thus, I rolled my own macro_rules! macro, bools_bitfield_struct! that generates const fns. The macro is specific for bitfields that are always bools, which makes things a bit simpler and have a cleaner API. And it also doesn't have to worry about being #[repr(C)] at all, which might makes things a bit simpler, too. Overall, though, it's quite a simple macro, so it seemed much easier to do it this. Since rav1d only used bitfields here, I kept the macro private in this file, but if we ever need bitfields that are all bools elsewhere, we can extract the macro. The c2rust-bitfields dependency is also now removed as it's unused.

[kkysen]

Mostly superceded by #501, but I'm leaving this open (and intending to merge in order) so the history doesn't get overly complicated.

[randomPoison]

I wouldn't normally want to add a custom macro for this, but since you're ultimately replacing it with the bitflags crate I'll approve this one as-is so that you can preserve merge order 👍