refactor representation of resolved pointer and optional types (#1552)

* refactor representation of resolved pointer and optional types

Adds a separate representation for optional types instead of
falling back to ast nodes in `Analyser.Type`.
Unifies pointer types into a single union field in `Analyser.Type`
which avoids duplicating some code.

* simplify `getFieldAccessType` signature

There was no need to take a Tokenizer because it can be constructed
from a source location.
There was no need to return the original and unwrapped type
because `resolveDerefType` can be used to get the unwrapped type.

* merge `resolveVarDeclAliasInternal` and `resolveVarDeclAliasUncached`