Stores one copy of each distinct JavaScript primitive.
For example, js_intern!("string") evaluates to a &JsValue but only
does the translation from the utf-8 Rust string to the utf-16 JavaScript
string the first time the expression is evaluated. Furthermore, strings
are de-duplicated across the program. So, any time js_intern!("string")
is used in the program, the same instance of the JavaScript string is used.