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.

npm-shrinkwrap.json

@@ -10479,9 +10479,9 @@
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     },
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-      "resolved": "https://registry.npmjs.org/yebool/-/yebool-1.0.1.tgz",
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+      "version": "1.0.2",
+      "resolved": "https://registry.npmjs.org/yebool/-/yebool-1.0.2.tgz",
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       "requires": {
         "jsbi": "^3.1.3"
       }