storageutil: new module for storage primitives (API) There will exist common code between storage backends. It would be nice to have a central place to put that code. This commit attempts to create that place by creating the "storageutil" module. The first thing we move is revlog.hash(), which is the function for computing the SHA-1 hash of revision fulltext and parents. Differential Revision: https://phab.mercurial-scm.org/D4753