Recipe Anchors

[starcraftman]

To allow easy updating of URIs.

In recipe: repo is string, rather than Fetchable. Or else, fetchable that retrieves information from a db stored in the recipe folder.

db in folder is a simple dictionary mapping. Per recipe directory. Maps name of recipes in the dictionary onto a single Fetchable/URI. So for instance, GIT_STABLE_ARC could map to a tar archive from linux downloads.