decrepit will contain algorithms that we really wish didn't exist any
longer. It won't be built by default in Chromium etc, but the code
will exist for crummy code that still needs it.

Change-Id: Ic307f5f0a69efe9e0a5fd54052f49d219e90dcdd

Initial fork from f2d678e6e8 (1.0.2 beta).

(This change contains substantial changes from the original and
effectively starts a new history.)