Standards.X509: Added generic parse_private_key().

Also adds variants of Standards.PKCS.{D,ECD,R}SA.parse_private_key()
that operate directly on ASN.1 sequences.
Nettle: Improve propagation of errors.

Use apply() instead of safe_apply() in lots of places. In addition
to improving the propagation of errors, it should reduce the stack
usage and improve performance, since it avoids the longjump contexts.

Also adds quite a few missing proxy functions, and improves the types
for some of the existing as well.