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
syntax val set "*(%1) = %2" prec 14 by specifying that
%1 has the same precedence as the