Improve memory management

[torpedro]

Currently the parse tree contains a large number of raw pointers to maintain the nodes. This means that we always need to explicitly delete all children upon deletion of a node. While we do have an automatic memory leak checker, the risk of memory leaks is still there and handling memory this way is cumbersome.

Possible improvements:

• Use std::string instead of char*
• Use smart pointers instead of raw pointers

We should measure the performance impact of using smart pointers. In general, I think the advantages of smart memory management will outweigh possible performance drawbacks.

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