as_bundle() should cache the bundle digest

[Gustavo Niemeyer]

FormulaDirectory.as_bundle() will create a formula bundle, and to make it unique it will use the digest of the bundle itself. This information (the digest) is slightly expensive to compute, and is necessary further on by the callers of the as_bundle() argument. The FormulaBundle should offer a way to cache this value so that it's not recomputed when something like e.g. FormulaBundle.get_digest() is called.