Introduce new abstract SequenceType

[HawkSK]

Proposition

Currently, all types representing a sequence (collection) extend ComponentType. However, also OptionalType is a ComponentType. A new intermediate empty abstract type uniting all sequence types is proposed, so that:

{ArrayType, ListType, ListTypeWithShared, OrderedSetType, SetType} <: SequenceType <: ComponentType
OptionalType <: ComponentType

Motivation

This will help in determining which types represent a sequence in an extensible way. Up until now, you could check this either by enumerating all seqence types (which will leave some out in the future):

if (type instanceof ArrayType || type instanceof ListType|| type instanceof ListTypeWithShared
    || type instanceof OrderedSetType|| type instanceof SetType) { }

Or by a more error-prone non-extensible, I dare say incorrect way:

if (type instanceof ComponentType && !type instanceof OptionalType) { }

The latter will cause problems in existing code in the future, when there is another non-sequence ComponentType introduced.

Tasks

• create SequenceType and change the inheritance hierarchy

• refactor all usages of ComponentType and OptionalType