addIntBig

    fn addIntBig(gz: *GenZir, limbs: []const std.math.big.Limb) !Zir.Inst.Ref {
        const astgen = gz.astgen;
        const gpa = astgen.gpa;
        try gz.instructions.ensureUnusedCapacity(gpa, 1);
        try astgen.instructions.ensureUnusedCapacity(gpa, 1);
        try astgen.string_bytes.ensureUnusedCapacity(gpa, @sizeOf(std.math.big.Limb) * limbs.len);

        const new_index: Zir.Inst.Index = @fromBackingInt(@intCast(astgen.instructions.len));
        astgen.instructions.appendAssumeCapacity(.{
            .tag = .int_big,
            .data = .{ .str = .{
                .start = @fromBackingInt(@intCast(astgen.string_bytes.items.len)),
                .len = @intCast(limbs.len),
            } },
        });
        gz.instructions.appendAssumeCapacity(new_index);
        astgen.string_bytes.appendSliceAssumeCapacity(mem.sliceAsBytes(limbs));
        return new_index.toRef();
    }