`EntryTypes` and `UnitEntryTypes` are inaccessible in validation crates.

I've just encountered a situation where I'd like to in which I like to validate that the `recipe_ah` of an `SignedEvmSigningOffer` points to a `Recipe` record. An imprecise way in which to do this is to attempt to deserialise the entry into the structure of a record. This approach is probably adequate but not flawless in general because the and struct type may overlap sufficiently to give a false positive. Therefore I would like to make a comparison on the records `EntryType` instead.

This isn't possible with the current crate structure because the `EntryTypes` and `UnitEntryTypes` enum are declared in the integrity crate, which is a dependant of the validation crate.

At the same time, we're also discussing separating differing functionality into zomes (see https://github.com/holochain-open-dev/holoom/issues/4). Integrity zomes cannot specify each other as dependencies, therefore we should devise a strategy that allows the enums to be shared between zomes without the zomes beings interdependent.