That should be done for Enrollment transactions.

We don't need a pointer here and this change makes this field compatible with
Transaction.Inputs which is useful in many scenarios.

Everywhere in this code prevHash == input.PrevHash, thus we can easily move
some common code out of the loop saving on DB accesses and
serialization/deserialization.

prevHash == input.PrevHash, so make less DB accesses and more real work. Fix
some bugs along the way:
 * spentCoins structure may already be present in the DB when persisting TX,
   there is nothing wrong with that and we shouldn't overwrite it
 * it's only used for NEO and only to check for claim validity. Thus, when
   processing claim tx the corresponding spentCoins should always be present
   in the DB

Which allows to use it for ClaimTX. Make it also a bit more efficient as maps
are expensive.

We don't need a map here, use simpler structures.

…etter

Claim transactions should _add_ claims scripthashes to the standard list, not
replace them. And this code is actually very reusable.

Refactor HaveInputsDuplicate() out of the core and Blockchain, it doesn't
depend on the Blockchain state. Make it more efficient.

Add doesn't change the variable state. Thanks to GolangCI for catching this.