Use bitfield-struct crate for PackedOperation.

Trying out a neat crate for Rust bitfields. The caveats are:

* Custom enums used in the bitfield must specify const funcs for bit
  conversion, and bytemuck's cast functions aren't const.
* The bitfield crate implements Clone for you, but does not seem to have
  a way to disable this. We can't rely on their clone, since for pointer
  types we need to allocate a new Box. To get around this,
  PackedOperation is a wrapper around an internal bitfield struct
  (rather than being a bitfield struct itself).

(Note: I'm not yet happy with this. Specifically, I think the
abstraction may be cleaner if OpBitField is defined entirely in terms of
raw native types and any reinterpretation / transmutation is done from
PackedOperation. Consider this a first pass.)

Cargo.toml

@@ -15,6 +15,7 @@ license = "Apache-2.0"
 # Each crate can add on specific features freely as it inherits.
 [workspace.dependencies]
 bytemuck = "1.19"
+bitfield-struct = "0.9.3"
 indexmap.version = "2.6.0"
 hashbrown.version = "0.14.5"
 num-bigint = "0.4"

crates/circuit/Cargo.toml

@@ -14,6 +14,7 @@ rayon.workspace = true
 ahash.workspace = true
 rustworkx-core.workspace = true
 bytemuck.workspace = true
+bitfield-struct.workspace = true
 num-complex.workspace = true
 ndarray.workspace = true
 numpy.workspace = true