Add constant folding

[JohannesLichtenberger]

No description provided.

[dvlpsh]

Hi @JohannesLichtenberger could you add a description for this issue as to what really is to be done?

[JohannesLichtenberger]

At compile time certain expressions can be evaluated already and thus, the AST has to be rewritten: https://gavinray97.github.io/blog/what-good-are-record-patterns

In brackit some rewrites (not constant folding) are already performed: https://github.com/sirixdb/brackit/tree/master/src/main/java/io/brackit/query/compiler/optimizer/walker/topdown