mail: let addressencode() / addrlistencode() return native strings Avoids conversion to "str" on py3.

mail: let headencode() return a native string This is to avoid conversion to/from str on py3.

revlog: deprecate direct `nodemap` access Now that all in-core user have been updated to the new API, we can deprecated direct access to nodemap. Differential Revision: https://phab.mercurial-scm.org/D7362