Reviewed-by: hjk

Well, it's not very advanced but it is probably good enough for now.

Type categories are now handled at semantic analysis time
so we don't need to track them at syntax analysis time.

DeclarationStatement wraps a single declaration, not a list.

Eventually we will need some way to locate an identifier's
definition, so add "lineno" to each AST node.  May want to
change this to "position" later.  The makeAstNode<T>() function
takes care of automatically decorating nodes with line numbers.