der: add EncodeValueLenCached wrapper

[dishmaker]

Sometimes, when encoding big DER structures, value_len gets called many times.

In order to limit amount of recursive calls, this wrapper caches the computed length,
until the value gets encoded.

[tarcieri]

Using a newtype for this seems pretty heavy-handed. Perhaps there's an alternative solution that can reduce the number of calls?

Do you have a specific example of where this is problematic? Could those types handle caching the length themselves?

[dishmaker]

I don't have an example. This just seems to be reasonable, where PKCS#15 nests objects recursively (example)
Actually, this newtype can be in a separate crate.