Part of structured approach for UpdateExpressions:
1) Expression gets parsed into a tokenlist (tokenized)
2) Tokenlist get transformed to expression tree (AST)
3) The AST gets validated (full semantic correctness) -> this commit
4) AST gets processed to perform the update
This commit uses the AST to perform validation. Validation makes sure the
nodes encounterd have valid values and they will also resolve values for
references that refer to item state or values passed into the expression.
