Fixes #264 (#266)

Fixes #253
Fixes #251

Multimaps and arrays that contained structs were previously
not correctly handled. When a new element is added to an
array or multimap that element must always be considered
new.

Previously the implementation would sometimes incorrectly
use a previous state of an element at the same index if
such element remained in memory as a result of shrinking
and then expanding the array or multimap again.

A similar problem also existed for optional fields in
structs where a field that was previously not present
becomes present and the fields state must be assumed to
be completely new in that case. Previously the state of
the field in some cases was incorrectly carried over
if the field remained in memory as a result of the field
becoming first non-present then present again.

This PR fixes all those bugs by correctly using "new"
state for new array and multimap elements and newly
present optional struct fields.

"seeds" directory now contains seed values that reproduce
the 3 bugs that are fixed.

Author: tigrannajaryan