(/=) SQLCodegen stub

First draft
(/=) Lexer : Distinguish numeric from string constants

(/=) propagate changes to the parser
(/=) update grammar
(/=) fix precedence in regular grammar definition : COMPARATORs have
higher precedence than UIDs, to make sure ‚like‘ is recognized as
COMPARATOR, not as UID
(+) start implementing codegen for selection

(/=) move identifier parsing code to selectionOperatorNode to share it
between passes over the syntax trees.
(+) dummy base relation operator
(+) selection using table scan operator