* `literal` is set to true if the expression's top-level is a JavaScript
literal (number, string, boolean, null/undefined, array, object), even
if it contains non-literals inside.
* `constant` is set to true if the expression is known to be made
entirely of constant values, i.e., evaluating it will always yield the
same result.
A consequence is that a JSON expression is guaranteed to be both literal
and constant.