Reconsider AbstractPathArgument hashCode()

Single-byte hashCodeGuard has not-so-nice effects in NodeWithValue and NodeIdentifierWithPredicates, causing them to have 7 bytes worth of padding per instance. Even NodeIdentifier has a 3/7 byte padding.

Eliminating hashCodeGuard in favor of treating hashCode == 0 as not present will have the effect of eliminating most of the padding:

NodeIdentifier goes from 24/32 bytes to 24 bytes, saving 8 bytes in the 64bit case.

NodeWithValue and NodeIdentifierWithPredicates go from 32/40 bytes to 24/32 bytes, saving 8 bytes for every case.