Convert backend code to ES modules

Bundling code now with Rollup for its flat bundling and tree shaking
support. Front end code still goes through Webpack for browser support.

lib/common/sat-solver.js

@@ -167,10 +167,8 @@ function unitPropagate(clauses, varVals) {
   return derivedVars;
 }
 
-function naiveDpll(clauses, maxvarnr) {
+export function naiveDpll(clauses, maxvarnr) {
   // variable values are 0 if not set, 1 if positive, -1 if negative
   const varVals = new Int32Array(maxvarnr + 1);
   return satisfiableAt(clauses, varVals, 0, 0);
 }
-
-exports.naiveDpll = naiveDpll;