initCapacity

    pub fn initCapacity(allocator: Allocator, capacity: usize) error{OutOfMemory}!Allocating {
        return .{
            .allocator = allocator,
            .writer = .{
                .buffer = if (capacity == 0)
                    &.{}
                else
                    (allocator.rawAlloc(capacity, .of(u8), @returnAddress()) orelse
                        return error.OutOfMemory)[0..capacity],
                .vtable = &vtable,
            },
            .alignment = .of(u8),
        };
    }