minor code improvement
slightly improve MakeBoolean()
avoid code duplication in Syntax class
simplify TokenToValueExpression()
Make true and false ValueKeywords, not builtin vars

Also add NULL. Add lexing and parsing support for
all three "value keywords".
add ET support for value keywords
fix value keyword parsing to avoid OutOfMemoryException
fix expression sequence parsing for newlines
support ternary operator
remove one debug output