rework dns_names helper, remove alloc req.

[cpu]

Description

This branch reworks the EndEntityCert's dns_names helper. Principally it:

• Simplifies the unit testing, and cleans up some misc nits.
• Reworks dns_names and the corresponding subject_name::list_cert_dns_names fns to be infallible, this better matches how we're ignoring invalid DNS names when validating a subject name since ignore invalid value validating dns name list #69.
• Removes the alloc requirement for the dns_names fn.
• Reworks the name ref types to use as_str instead of a From impl
• Removes the GeneralDnsNameRef type (with the potential to restore as future work c.f. Restore GeneralDnsNameRef, use in GeneralName::DnsName. #183)
• Moves the subject_name::list_cert_dns_names free-standing fn to be associated with Cert and named valid_dns_names.

Resolves #46, replaces #50