Fix race conditions in Keyring::read

[josephlr]

Actually fixes #21

It turns out most of the test issues were caused by the following race condition in Keyring::read:

• The key can be removed/invalidated between reading from the keyring, and reading the description from the child key (in order to determine its type).

We fix these issues by checking for ENOKEY when calling key.description() and skip that key.

Note: clippy is still failing, (resolved with #26)