Implement CASE operator

This also modifies the behavior of AND/OR operators to strictly return
true/false/null whereas previously they could return the value of the
first truthy/false operand. The purpose of this change is to maintain
expression equivalency before and after minimization.

package.json

@@ -34,7 +34,7 @@
     "parsimmon": "^1.13.0",
     "seedrandom": "^3.0.1",
     "yaml": "^1.6.0",
-    "yebool": "^1.0.1"
+    "yebool": "^1.0.2"
   },
   "devDependencies": {
     "@rollup/plugin-commonjs": "^14.0.0",