Use internal rather than scala-based heirarchy for types
Correct behaviour of merging type sets
Rename iteratedType on AnyType to legacyIteratedType

And use innerType on CollectionType
Stop TypeSet inheriting from Set[CypherType]

This allows for a more specific interface without the need to implement
all the methods of Set manipulation
No coerced mergeup
Add indefinite sized type sets
Use indefinite type sets in semantic checking

Resolves multiple type related issues, steming from the incorrect
usage of mergeUp for type constraining, and the use of AnyType() to
represent all types.
Remove DiscreteTypeSet
Fix type calculations of +
All expressions initially have all possible types
Track specified and expected expression types separately