Add support for `day` and other timestamp keywords in DF

[DanCodedThis]

Post-process the AST:

• Parse the query string into statement
• Use the visitor pattern to post-process the statement
• Change the Ident in the function calls to appropriate AST type, which wouldn't confuse DF

[DanCodedThis]

Probably should drop the idea with a macro, as it could lead to confusion

[DanCodedThis]

Normal VisitorMut should be enough