support enum variants having specified values

[Gankra]

The kdl-script parser supports enum variants having an assigned value (IntExpr)

abi-cafe/kdl-script/src/parse.rs

Lines 273 to 280 in 6ba6865

	/// A variant of an [`EnumDecl`].
	#[derive(Debug, Clone)]
	pub struct EnumVariant {
	pub name: Ident,
	/// Optional value this case is required to have
	/// in its underlying integer representation.
	pub val: Option<IntExpr>,
	}

But the type system throws it out.

abi-cafe/kdl-script/src/types.rs

Lines 259 to 264 in 6ba6865

	/// An enum variant
	#[derive(Debug, Clone, PartialEq, Eq, Hash)]
	pub struct EnumVariantTy {
	pub name: Ident,
	// pub val: LiteralExpr,
	}

---

Once the type system supports it, the backends need to be changed to use it:

abi-cafe/src/abis/rust/declare.rs

Lines 209 to 216 in 6ba6865

	writeln!(f, "#[derive(Debug, Copy, Clone, PartialEq)]")?;
	writeln!(f, "enum {} {{", enum_ty.name)?;
	f.add_indent(1);
	for variant in &enum_ty.variants {
	let variant_name = &variant.name;
	writeln!(f, "{variant_name},")?;
	}
	f.sub_indent(1);

abi-cafe/src/abis/c/declare.rs

Lines 250 to 257 in 6ba6865

	writeln!(f, "typedef enum {} {{", enum_ty.name)?;
	f.add_indent(1);
	for variant in &enum_ty.variants {
	let variant_name = &variant.name;
	writeln!(f, "{}_{variant_name},", enum_ty.name)?;
	}
	f.sub_indent(1);
	writeln!(f, "}} {};\n", enum_ty.name)?;

---

Arguably IntExpr shouldn't be used here, since it artificially clamps to i64. In general we don't have a good story for talking about arbitrary primitive values that rust (and kdl) can't express. But i64 is good enough to start with.