Precedence of extraction driver symbols (fix #223)

This MR allows to define precedence level and associativity direction of syntax val driver symbols. Using this information in the extraction removes many useless parentheses.

TODO: add a way to define the precedence level of subterms inside driver definitions. This would allow to remove the parentheses around %1 in syntax val set "*(%1) = %2" prec 14 by specifying that %1 has the same precedence as the * operator.