As a partial step towards #1322 , possibly building #1319 nicely, and generally improving link handling, we need a "link status bear" or "link bear" which extracts all links from a file and stores information about those links in a re-usable manner. Then InvalidLinkBear and MementoBear and other link analysis bears can re-use that information instead of duplicating analysis, especially avoiding re-fetching (with HEAD) the same URLs multiple times.

This will be like AnnotationBear; other bears will list this new bear as one of their BEAR_DEPS.

In the future, this new bear would also use AnnotationBear to gain language-awareness so that link extraction can be more context aware.