Explicit requirement renaming

[eikopf]

When the identifier of a requirement changes, Tracey treats it as a new requirement rather than an updated version of an old requirement. This prevents us from treating references to the old name as being stale rather than orphaned, and might make users wary about changing a name for fear of losing the associated data.

The proposal here is to add support for recording previous requirement identifiers within the spec itself, by adding a previously requirement to the requirement syntax as follows:

r[auth.login+5 previously=authentication.login,original.login]
Users MUST authenticate with a valid token.

This will require an update to marq to support a new attribute in requirements, but shouldn't strictly need any changes to tracey to work. Once marq supports this, we can update tracey query to handle previous identifiers properly.