backwards incompatible change to RFC822Name (#3953)

* backwards incompatible change to RFC822Name

During this release cycle we decided to officially deprecate passing
U-labels to our GeneralName constructors. At first we tried changing
this in a purely backwards compatible way but get_values_for_type made
that untenable. This PR modifies RFC822Name to accept two types:
U-label strings (which raises a deprecation warning) and A-label strings
(the new preferred type). There is also a constructor for RFC822Name
that bypasses validation so we can parse garbage out of certificates
(and round trip it if necessary)

* whoops
5 files changed