Add interrupt to parser. Add elaborator to parser. Add placeholder support in the scanner.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Add check_interrupted 'macro'

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Add support for creating meta-variables in the parser.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>