Internal enhancement tracking the long-standing wish to have a dedicated API for constructing single recipients and recipient collections.

Motivation:

• centralize recipient name/address parsing for strings, InternetAddress instances, existing Recipient objects, fixed/default names, and delimited address lists
• Unburden the EmailBuilder API
• make recipient construction easier to reuse outside the large EmailBuilder surface
• unblock Encrypt mail with multiple certificate #297 by allowing per-recipient metadata such as S/MIME certificates to be represented cleanly

This is separate from #297, which tracks encrypting mail for multiple recipients with different certificates.