> Is the length always the number of bytes? I was under the impression
>> it's the number of elements in case the value is a list/vector.
>
> No, it's always bytes.  See <https://en.wikipedia.org/wiki/X.690>, which is
> a handy summary, though hardly suitable as an explanation from scratch.

That troubles me a bit. It means that when writing data, you need to
know in advance how much you are writing (even for complex nested
structures of heterogeneous data, not just for strings). Effectively you
have to either:

1. Two passes over the data (in the first pass, count the total length
without writing anything; in the second pass, actually write).

2. Write a dummy value for the length, then seek back the stream and
patch it once you know the true length.

3. Write objects recursively in a stack of temp buffers, each layer of
buffers being concatenated one layer up.

None of these are ideal. It allows the reader to skip past uninteresting
objects very quickly, but that's not much of an issue nowadays.