Add store/restore functionality to digest::Context about ring HOT 2 OPEN

I don't have a problem with adding this, but I don't have cycles to do the work on it, so somebody would need to contribute it.

• The format of the internal state should be opaque to the API. The API should export/import a slice of bytes. This means we need to pick an endianness for the bytes. I suggest little endian, since that's less work for most targets.
• We shouldn't try to serialize digest::Algorithm or ``digest::AlgorithmId. Instead the user should pass in the &digest::Algorithm`. The user will need to be responsible for keeping track of what algorithm goes to which bytes.
• The APIs should be named "....less_safe()`.
• We should have tests that exercise every algorithm, in the same style as the existing digest tests.

from ring.

PR - #2081

from ring.