Implement Line-Based Feature Traces with ASTs (After Issue #54)

[pmbittner]

This issue depends on issue #54.

When you derived interfaces for feature traces and extracted the marker logic to the core library, it is time to implement the new AST based representation. Therefore, we should keep things simple first by only considering AST<LineGrammar, String> instead of the generic case AST<G,V>. So the task is:

Provide implementations for all interfaces you derived in issue #54 using AST<LineGrammar, String>.
Afterwards, all interfaces should have (at most) two implementations: One for the markers and one for your ASTs.