Expression visitor/walker & node state rewrite
This PR
- Adds the
ExpressionVisitor
andExpressionWalker
traits to the parser - Removes the non-compliant/broken
Ord
implementation from AST nodes and tokens - Rewrites
node_state
a little bit based on the two aforementioned changes and removes theNodeState
structure- Additionally, it is no longer possible to get an associated type for temporal operators, as this is now considered implementation-dependant