Eliminate differences between `Ast` and `Tree` ✨

[kazcw]

Currently the frontend has some small differences in its Ast representation from Tree. These differences relate to the dot operator, the negation operator, and operator sections. The interfaces should be synchronized. In the dot-operator case, the distinction should be moved into the parser; this will probably enable some simplification in TreeToIr.

[hubertp]

/cc @JaroslavTulach since that will somehow affect TreeToIr