ASTStore: Add UD functionality

[jubnzv]

We could store UD relations in the AstStore by introducing some kind of definition IDs that refer to the node IDs present in the AstStore.

Since we don't have any special IR nodes for this (#74) yet, we could design it in AstStore:

• Store function/method arguments in the AstStore, as we need them to define local variables
• Create a type alias, similar to rustc's DefId
• Create a mapping from most of the AstId.id to these DefIds
• Fill the mappings using AST traversals at the top level and UD analysis at the function level
• Create a public API to access this information

Depends on #186; otherwise, it will be unreadable.

[jubnzv]

Also useful in UnusedExpressionResult (#83) since we can combine UDs with ASTStore.getReturnTypes()