Single checksum parameter that includes contact ID

Proposed by Stoob here in CRM-16448 (https://issues.civicrm.org/jira/browse/CRM-16448?focusedCommentId=85030&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-85030):

I have an idea to add a new (additional) Checksum token that consolidates the following into a single token for easier use:

&{contact.checksum}&cid={contact.contact_id}

Since this is consistent across all checksummed uses why isn't it just one button?
We should keep the old token as is, we don't want anyone's existing templates to break. But a new token could be easier than ever to use. Checksum is a cool feature and marketplace advantage that I'd say less than half, maybe even less than 20% of Civi users even know about