Substitute non-const visitor by const visitor where ever possible

[muellch]

After #617 was resolved by #1094, only the ASTStringifier now uses the const visitor interface.

For every other visitor implemented, one should check if it is possible to switch to the const visitor interface.

This task cannot be started before #648 is completed, since a const variant of the expr::getData method is required to make most visitors const, which cannot be provided as long as data is lazily initialized in the getter.